Question

why do you capitalize the seasons in some sentences but in others you dont?

Answer 1

Seasons are usually not capitalized. The exception if the season is followed by a noun and the term is a proper noun. Both these sites use Spring term or Fall semester as examples of exceptions.

http://oregonstate.edu/dept/eli/buswrite/capitalization.html

http://owl.english.purdue.edu/owl/resource/592/01/