Why do Democrats try and force everyone to be Democrats?

Both my parents are Democrats (seems like die heart Democrats....you'll see what I mean as you read on), and they are trying to force me and my brother to be Democrats (believe, vote, whole thing). My brother printed out and filled in his voter registration before he left for college (he in a different state for college), and he put he was Republican. My parents are mailing it one day this week, and they both had a fit and wanted to scream at him if he were here. Apparently they told him (that's what they told me anyways). They said they hadda change it to Democrat cuz "we are a Democrat family and we can't have anyone not Democrat, and we are afraid if someone knows our child isn't Democrat cuz our reputation with the Democrats for all these years." I'm filling it out today and I'm a Liberal, and now I'm afraid to send it like that cuz they making me show them just so I don't put anything but Democrat. They also said my brother "didn't know what he was doing by filling in Republican," they said my brother didnt know you have to leave it blank, and if we need help from mayor and whoever is in office they look to see if you vote and if you have registered party affiliation cuz that matters to them if you vote and what you are (which is BS to me....no one really cares).

Why can't they just let anyone believe what they want? Apparently according to them "at this young age we don't know anything and they trying to guide us."

To me at least it don't sound like guiding when they forcing us to be Democrats and being cavemen people over it.