I think it’s merely expressing a wish for someone to overcome whatever ails them, not a command.
Not rude.
…and yet another thread about “choosing” weather or not to be offended. How long before its rude because he DIDN’T say anything?
“Feel better…Or else.” :mad:
Seriously, this is a concern? 
Not even 60 minutes ago I texted back to a flu-ridden friend of mine “get well soon! Rest and liquids! Maybe try frozen juice pops. :-)” (That’s verbatim.)
Well, she didn’t sound irritated with me.
Are you her?
“Feel better or I’ll make you feel worse”? 
I just pull a Patton and bitch slap ya if you look like you need a picker upper.
How do people say it to you so that it sounds offensive to you? In my experience, it’s always said in a tone of voice that couldn’t possibly be interpreted as a command.
My 1.2 annoyance in this vein is “Be careful” It strikes both as a command and an unnecessary redundancy.
“Wow, look at the storm! I still have to drive home. Oh, well, here I go…”
“Be careful!”
“Um, okay. Hadn’t thought of that. I wasn’t going to be, but since you insist…”
ETA: I know it’s always intended as “I hope you will be so careful as to not come to harm” and I appreciate it as such. But my over-literal inner pedant gets loud sometimes.
It’s different when the person may not be aware of the danger – “That front sidewalk doesn’t look icy, but it is, so be careful.”
Reasonable: Person coming down with the flu who just keeps working till he’s ready to drop: “Will you FFS take care of yourself already?”
Redundant: Person going home for the evening; everyone in the office tells him to “take care of yourself, now!”
ETA: 7777 posts! Is that like coming up all cherries?
I understand that the form is generally a command, but as others have said, it’s common in other ways too, like “have a good day” or “take care”. One of my favorite ways to say bye to people is “Have fun!” which works great if they’re doing something they enjoy, and is just amusing if they’re doing something mundane like going to bed.
I think the issue here is just that you’re only really going to hear “feel better” in a situation where you’re not feeling well, so you’re more likely to be a bit more on edge and sense a bit of intention that isn’t there. Why not just assume the missing part is “I hope you feel better” rather than “You, feel better!”
That’s incredibly easy to explain: It’s irrational.
And unlike some people in this thread have said, it doesn’t really bug me. I get over it 0.0000002 seconds after it’s said. It’s kind of like someone playing a wrong note on a piano.