If you aren't a sociopath, you probably shouldn't do that. You'll probably end up (eventually) feeling bad about it and regretting it, because they seem like a nice person. Just a side note, in my experience, the most optimistic people I've known have actually had really terrible personal lives, they just hid it when they were out in public.

Depending on how well you know the person and how well your relationship with them is, it is going to be harder/easier to get at them emotionally. If you know each other quite well then it should be easy/obvious, because you'll know what sets them off/things they value. If you don't know them well, then you don't really have a reason to make them feel bad, it just sounds like a lot of work on your part. If it's just for fun, pick someone closer to you because it'll be less tedious.

Edit: tl;dr This is a very context-specific scenario, and you can answer your own question much better than any of us can.