There's no stigma associated with self improvement. Say, wanting to be more confident.

The sort who can't last five minutes without bringing up how much they improved will find plenty of stigma.

There's no stigma associated with wanting to help people.

Provided you don't become a self-righteous ass about it.

Maybe it's because all of those other things are narrow enough that it's not seen as an attempt to be "better" than others. But since rationality is so general, it is seen as an attempt to be "better" than others.

It's an attitude thing. People will perceive an attempt to be better than others if the individual starts acting the part. Socrates made a lot of enemies with his habit of going around correcting flaws in people's thinking.