It's a defence mechanism. Actively improving yourself makes the rest of the group look and feel bad [because they aren't]. They don't like those negative emotions, so they try to equalize. But it's much easier to equalize you to them, than themselves to you.

So they make fun. They passively bring down your efforts. They're much too nice to do it actively. And provide plenty of excuses for you to fall off the wagon.

BUT!

This is also a test of your resolve. If you can't do something against your friends' and family's will, how do you know you actually want to do it?

At least that's how I look at it. If I'm not confident enough in my choices that they would stand up to the scrutiny of my friends and family, then my choices need some more polish.