Why does marijuana gets me aroused?

It's complicated. The chemistry may be complex, but the short answer is: mj is a drug. Drugs can do many things to the mind, so it isn't at all strange that hypersexuality is one of those effects.
Trying to think. This through. If you are normally a structured person, if may allow you to relax enough that your libido is no longer under tight reign. However, there can be serious mental health problems for some people who use canabis.