I'm not going to get excited about my friend getting engaged
Why aren't you excited for your friend?
I don't get any bonus out of it.
It is rude not to show any enthusiasm for your friend after they get engaged. This is an exciting moment for them and you should be supportive.
I'm not going to be supportive.
This moment is about your friend, not about you. Don't you want your friend to be happy? If you congratulate them and show your joy, you'll both become better friends.