I never pledged - I rushed a couple frats, but apparently I didn’t give the right impression, and I made through college just fine without that part of college life.
If you did pledge, I’m curious about your motivations for doing so. Poll to follow.
I needed addresses to bill all those millions of “12 free records for a penny” accounts from Colombia House.
I was kind of the same way freshmen year. I rushed a few fraternities with some of my hall mates, but none of them really clicked. My sophomore year, I decided to start playing ice hockey again and I became friends with one of my team mates who happened to be rush chairman of the house I eventually joined.
Mostly I just liked the guys. 20 years later, I’m still close with a bunch of them
Got to live in a nice old house with a cook (for less money than a dorm room & cafeteria food) where you could basically do whatever you wanted to, a built in group of of close friends to do all sorts of stuff with, (if I’m being honest) completely barrier-free access to alcohol while underage, rotating parties at other fraternity/sorority houses pretty much every Thu-Fri-Sat night whenever you wanted to go, interfraternity sport leagues, etc.
My mom originally guilted me into going through Rush Week. She came from a huge family and couldn’t afford it, though she had a lot of friends who were in sororities and would have been in one if she could have.
And I did end up enjoying it, for the most part.
Most of my friends from my dorm floor ended up joining the same fraternity - so I figured I might as well. My campus was 30%-40% Greek, and a greater percentage of social functions happened through the Greek system. When I entered college I never even considered joining a fraternity. Ten years out, I’m still great friends with a lot of people I met through the experience.
Two close friends from high school had joined a particular sorority and when I attended the same school, it seemed like a natural thing to join them. I spent Soph and Jr years in the house and somewhat enjoyed it, although I found all the rules and restrictions and social expectations needlessly grating. After my two friends graduated at the end of my Jr year, I committed the ultimate betrayal and left the house. In retrospect, I wish I had toughed it out and stayed the final year.
I joined a professional music fraternity, albeit quite a social one. Mostly I was in it for the singing. Lots and lots of singing.
QFT!
There was an informal competition to see who could come up with the most outrageous name.
I won. 
Let’s see…
I got to be around people like myself. My college campus was much more “multi-cultural” than my high school campus. I wanted to hang around people who reminded me of my high school peers. I didn’t have to worry about running into nerds, thugs, hippies, or really religious people at the parties we threw.
We had the most power on campus, because we had the most money. My family didn’t have that much money, but many of my brothers did come from “good” families. So did the sorority girls. I dated a few, and it was a delight. One of the girls grew up in a house that had a literal fountain in the foyer!
It felt cool walking around campus with those Greek letter shirts.
Visiting different chapters of my fraternity across the country. It was like having a huge extended family.
Of course, I’m a different person now. I think nerds and thugs are cool these days. And, I couldn’t care less about how much money people have. Also, I’ve found my sense of belonging in my “biking and brew” meet-up group. But, when I was 19, Greek life was the sort of thing that appealed to me.
I went to a rural small-town high school that was 99.x% white, and going to college and being around people from vastly different backgrounds who were into vastly different things was exciting. I couldn’t have imagined trying to stifle that.
I didn’t pledge a fraternity because I really didn’t care to. I had friends who were Greek, but I never regretted passing on the whole deal. I had plenty of friends through my major and through meeting friends-of-friends, we had plenty of fun and plenty of parties and got plenty of girls the “independent” way. 
Do you actually have to formally quit or do you just stop showing up for things? Why do you regret quitting?
I may have phrased that a bit too dramatically (I tend to do that). I didn’t quit, although you are able to do that if you desire, I simply deactivated because I didn’t want to live in the house anymore. If you don’t know how that works, when you deactivate, you don’t relinquish your membership, you are still a sister for life. In a way it’s like a leave of absence. I moved in with my fiance because I was young and in luuuuvvv. I wish I had stayed in the house my final year because there are some real perks to being a senior in a sorority and because I felt as though I burned bridges with some of my sisters by rejecting their company in such a public way. I could have waited that additional year and I think I should have.
I was never a big fan of communal living and I’m fiercely independent. I was not comfortable with the overwhelming ‘girlishness’ of a group of women living together, but I did like many of the individual girls in the group and feel bad that I let them down.
I had never planned to rush or anything, but my junior year a bunch of the guys in my dorm all happened to be in one fraternity, and I hit it off great with them, so I just gravitated there.
Question from a non-Yank: what does “rushing” and “pledging” mean?
Speaking from my experience (though I think the process is generally the same at most US colleges and universities):
“Rush” is the period at the beginning of the school year when the fraternities try to recruit new members - they will visit dorms, hold various parties and events (some open houses, some invite-only), etc.
The fraternities eventually offer “bids” (invitations to join) to those rushees they are most interested in…on “pledge night” the rushees declare which house (if any) they have chosen - an initiation period follows, at the end of which the “pledges” become initiated as full-fledged members (“brothers”).
Thanks. That helps.
“bids” = “tap night”, then?
Well, I associate the term “tap night” more with “secret” societies like Yale’s Skull & Bones, and I’m not quite sure how that all works - for us bids were written invitations (much like wedding invitations), and pledging was a public process - on “pledge night” (a Friday in late September) the rushees would gather in an auditorium on campus and meet with representatives from their chosen frat, who would escort the group of pledges down fraternity row back to their chosen house where they’d be welcomed with cheers, songs, drinking, etc., and the night would turn into a big party.
I made friends with many of my future brothers by playing Dungeons and Dragons at the house for a couple semesters before they asked me to pledge. It made it easier for all of us since I was acquainted already, and I stay in touch with many of them to this day.
Fraternities typically have a financial obligation. So if you stop paying your dues, you effectively quit the fraternity. Although individual fraternities might have rules about how much participation is required.
In our fraternity, as well as many others, just leaving the house typically didn’t mean quitting the fraternity. Many fraternity houses didn’t have capacity for every brother to live in it. A lot of seniors also tend to move to off campus houses because they get tired of the communal living.