I feel like that, and at Christmas, too.
I think it's because although I am happy (most of the time!) with my adult life, I miss the extreme happiness I felt throughout my childhood and most of my teenage years. I miss the excitement I used to feel before my birthday, when everything was about me and my family would come round, and everything about Christmas that was so exciting, bright and hopefully. It was just joyous.
Then things change, you get a taste of life and responsibility. Christmas isn't the same when you're an adult, it's bloody hard work. My parents split when I was 24 so things are quite awkward now. My mum fell out with her brother which (after lots of things happening) means we don't see that side of the family at all. My partner has never really celebrated birthdays and Christmas in the same way I did, so he doesn't understand, and pretty much sees them as another day anyway (though he does like the drinking and partying).
I think I get wistful, thinking how happy I used to be, and how sad it is that I will never, ever feel that carefree happiness like a child feels. But when you're a child - obviously - you don't appreciate it. In fact I specifically remember constantly wishing I was older!
I'm sad because I CAN'T get those feelings back, and I so very much want them. I also get a little bit sad because I'm unlikely to have children of my own. My partner has a son and doesn't want more kids, and although I like the 'idea' of being a mum, when I think about it, I think I'm probably too selfish. So it's not even like I can make Christmas magical for a child, either.
I know that probably doesn't help you, as it might not be the same reason, but it's how I feel.