I'm upset that a younger friend of mine is dating someone who makes fun of me
What do you think made your friend do this to you?
I guess it's my fault for not being nice to him
I think your friend could have found a better way to handle being upset with you over being mean. I doubt they are dating someone to spite you they might have a real connection.
At least this is proof that caring about someone is a waste of time and I should be more cynical.
It is wrong to blame other people for your problems. You should try to handle them like an adult and respect your friends choices.