Sometimes it is the things that don't happen, sometimes the things that do; words uttered, words left unsaid; letting go and wishing otherwise, not letting go and wondering if that ought be the way. All that matter, that don't; that should matter, that shouldn't. There may have been some hurt, there may not; there may be some regrets, but then again maybe not. Too much may have been expressed, too much may have been suppressed.
There may be goals, strongly desired, yet unreachable - for physical and mental limitations, for fickleness and uncertainty.
There are nights sleep might soothe the mind's incessant torture, but rest would not come easily. There are days music might lessen the gnawing pain in the heart, but the songs intensify the melancholy. There are moments tears might relieve the emotional turmoil, but tears will not form.
Maybe it is a lot of things, maybe it is nothing. Maybe it's just the weather.