What’s the reason some people act like that?

I just wanna understand why do some people who want to be close with you, and keep trying to be your acquaintance while they are the first to laugh at you if you fall and when you ask them about something they make you feel ridiculous. You are also the first who they run to when they are in need or in a tight situation.

I wish I can quickly smell them and kick them far away from my life! So I wanna understand so I can better detect such a people.