Hi Summerlove and welcome. You may find the answer to your birthday question varies. I've had different answers given over the years. Some of them include:
"We don't believe in putting anyone on a pedestal." (My reply to that is "Then why do you celebrate weddings? Isn't that putting the bride and groom on a pedestal?" I didn't get an answer to that.)
"The first birthday present was John the Baptist's head. So we don't celebrate birthdays because it is tied to murder." (Uh, ok but you could probably say that about a number of things. I've never been to a birthday party where the person received a severed head.)
I think I was told it has pagan origins one time as well. Yeah, so do a lot of other things like wedding rings and neckties. So what.