I guess I'm just not around that many people who don't know me well or something. I don't get a lot of that kind of crap. Somehow I seem to emit an icy/spacey vibe or something when I'm not feeling chatty and when I'm really not feeling chatty and I have to be around people, I pop in my earbuds and most people take the hint. Around here, people are friendly, but it's rare that they are pushy about it.
OTOH, I'm not all concerned about being sincere or whatever. If someone is asking for affirmation in some way, I try to give it to them. I don't have to feel like it. I mean, to me, not giving someone who needs affirmation what they need because I'm not feeling it would be like not feeding my kids because I'm not feeling nurturing today. If a relative or friend needs something and I have it in my power to give it to them, they are going to get it. It's just not a big deal and it's not about me. It's about them.
Now if they keep at it and keep at it eventually I'm going to tire of it and play dumb like I'm not picking up the signals. They can decide if they want to seek affirmation elsewhere or they want to keep talking to me. *shrug*
As far as showing enthusiasm goes, I'm a quiet person and I'm not bubbly. I am genuinely pleased and happy for people when good things happen for them or they accomplish something, etc and I will show it, but it's going to be "Are you serious?? That's great! I'm so happy for you!" Not screaming or jumping or anything, though if it's a big deal like a much hoped for pregnancy, I'll probably cry.
To me, people who are overly concerned with being honest, etc with people all the time just end up dumping their crap on everyone for them to have to deal with instead of being considerate and using a little restraint. Really, most people do not care what you honestly think or feel. They are just trying to get through their day and do their thing. Your opinion, my opinion? Not that important.
That doesn't mean I will not respond in kind if someone is being a bully or whatever. My default is generally cooperative and polite. If that is met with something above and beyond, I will probably go above and beyond myself. If someone is snotty or rude, I'll try to make sure it's not just a misunderstanding first, but then I figure that is the tone they want to set for the encounter and act accordingly. They tend to back off and behave themselves then, which is just more pleasant for everyone.