Back to California is a perception of a fading relationship fling.  You hit it off, it's great, then it starts to fade.  'California' is a metaphor for 'going back to your perfect life' whereas 'Watch the seasons change' is a metaphor for 'watching her fade'

Back To California

Seventeen is a fictional story about a 17 year old boy that tries to commit suicide after a serious breakup.  
I won't spoil the story for you but the moral is to "Keep fighting. Even when you feel like there's nothing worth living for, don't give up."

The first verse is from her perspective, and the second and final verse are from his perspective. 
The chorus is from my perception of the situation.

Greg Blucher Official