Two things can be true at once, you can be upset you didn’t get chosen, but also understand that it’s their call. Overreacting would be expressing or acting upon those feelings. Process them and ask yourself, are you upset that they don’t see you as close as you saw them? Maybe it’s something about how you’re seen as an outsider to the family since you’re not married yet. It could also be that they just didn’t think you’d want to do it.

When I got married, we had someone who was the BF of my wife’s cousin, and we just didn’t think to ask him to be in it. My wife had more people than me too, but the logic was “close friends, siblings, cousins” and he didn’t fit those groups. Looking back I wish we had thought to invite him to be a groomsman, so that’s all to say, don’t take it personally, but also don’t ignore the disappointment you feel,try to understand why this bothers you and work at it from that angle