[heedlessly2]

My guess is that during the summer and spring, you see other people outdoor enjoying activities. If you are a lonely person, this can be discouraging that everyone around is having fun but you're not.

In the fall and winter, you have an excuse to stay in. Everybody else is doing the same, so you're not missing out on much.

Plus part of it is academic. For k-12 and university, you see your 'friends' and 'acquaintances' very often. During the non-academic season, you don't see people as much which can be isolating.

Plus summer and spring, has the expectation of "you're supposed to be happy". So if you're not, then this can be devastating. Winter everybody has this "we're all in this gloomy mood together".