This may seem ridiculous to ask, but why can't people just be honest about themselves?

The reply lies in your query itself.
To add to them, perhaps they get so much conditioned being dishonest that they seem to forget the way to be honest.

I think you answered your own question

I would have to say it's everything you said. And it's also a sign of weakness. I hate liars they are the worst kinds of people and should be cut off right away because it tells you exactly what type of people they are

It probably depends on the type of dishonesty (exaggeration, outright deceit, simply wanting to be agreeable, a truth one cannot admit even to oneself and so on)

It could be any of those. Sometimes even worse, they could just be playing right from the start

We all see things differently I guess. Some people have no idea how others may feel only themselves.

I'm guessing most want to make the best impression as possible, so make it sound good, who knows for sure

Perhaps because the truth can hurt sometimes.

[c=#BF0080]I'm definitely honest about myself.[/c]

I always am. Despite the rejection

Did something happen?

It is all of those things, I imagine.