Yeah, it also frustrates me when people ask this, because unless it is strictly a situational depression, then you can't really answer that. My boyfriend asked me that once, and I basically told him it was equivalent to asking me why I had a mental illness. That got him thinking and he apologized for the question. He only wanted to help, but it made me feel kind of bad.
I got way worse on Lexapro (mixed episode), but was not on anything mood stabilizing at the time. Please let your doctor know if it is making you worse, as it almost put me in the hospital. The shore always makes me feel a little more alive, so I hope you have a great trip!