After living with two noisy people below me for a year, I know what you're going through.
The first was actually a husband, wife and 2 yr old. The husband had a nice system...for a club and would play punk music ALL day long! It was so bad that we could make out the WORDS of the song.
We called the front office and complained about them, they counteracted and said it was because we stomped around all the time. Finally we called the police on them. Apparently the dispatch recognised the apartment number and said "this is for apt ####, right?"
They FINALLY moved out...we came to a truce because of the kid (we just began to stomp because we were sick of going downstairs all the time). Turns out we were scaring the kid (nevermind that IMO the father was ruining the child's hearing!).
We got two weeks of beautiful golden silence when suddenly PARTY GIRL arrived. She would pound ice on the counter until 4 am...5 nights a week.
Long story short we decided to rent a house and its been great since.
But to answer your question, why people must be loud? Thanks to the lovely invention of Rock n' Roll and well IMO the hair band in the 80s didn't help (thanks to my brother blaring his music when I was 10, I now can officially say I can fall asleep to Motley Crue
)Plus, the technology is affordable now. IMO I think that apartments should outlaw the stuff. It makes the living conditions worse and probably causes the police to go nuts!