Is it a Good Thing?

When a guy and a girl go out for about a year, and then start sharing personalities.

i.e. My Bf was an uncaring wreckless person, but now he's much more cautious. I use to say "excuse you" after he burps.. but now he knows to say it automatically. I use to tell him to "be nice" and now he doesn't have to the heart to tell people off. He's much more self concsious of how he looks and what not when he didn't care before.
I, on the other hand, stopped caring about anythign and everything (other than him). I stopped controlling my anger and voice out my opinion when I feel its needed. I've gain some self esteem because of him, but I haven't completely become him.

Makes sense? Is it a good thing? Would that kind of relationship lead into disaster??

no thats good!!!! you both have good qualities, and are sort of sharing them, therefore making your relationship alot stronger =)

that can be a good thing ..unless the personality of the other person is bad....for example...im not going to lie when it comes to sex men and women i like what i like and i always get what i want! a girl i was seeing at the time started to feel this about herself..she was pretty popular among mnay crouds so i never thought about it...but when we broke up she ended up dead becouse of it....everyone...remember who u are not who other people are

Eh? Depends methinks.
In your case you two are just growing on a personal level together (good thing btw), and not neccessarily "loosing yourself" in this relationship.

When you two start colour coordinating, and matching as to what you wear.
Then ....