Why shower? Why?

Why does it seem that when you've just started to relax in the shower( I do not take baths anymore unless my body hurts) that all the hot water runs out????? Can someone answer me that? Everytime, either the hot water runs out or my parents beng on the door telling me get out( they think me and my long showers are the reason they're gas bill is so high but really who are they kidding!!!). I just got out the shower a few minutes ago so that's why I feel so passionatley about this subject.