My parents are what stops me. I know I could easily handle another tank, but it's their house, their rules.
Luckily, after Apollo died, my mother eventually agreed to let me get Odysseus (named today after 3 weeks with me, lol!), who looks pretty similar to that boy but is a butterfly, and slightly less yellow. My only reason for wanting him was that he absolutely pulled my heartstrings when I saw him - love at first sight. He was in a good petshop with good care, a litre and a bit to swim in, mental stimulation, clean (if slightly cool) water...but he was so beautiful, and the only HM I've seen in my city. Even his $26.50 price tag couldn't put me off (especially since the lovely shop owner let me have him for $11.50!).
It's the same with rats, though. I see a rat as the RSPCA, and I just think, I could give that little boy the most amazing life, and I know I have the money, time and love...but my parents say no.
Life will become more difficult when I graduate and leave home, I think...less money, less time, but more freedom to rescue every little critter I see...