You don't give much details so its hard to tell, but if it is for example a person or animal who died, it could be because it is only when you think of them that you feel (or realise more acutely) that they are not there anymore and you miss the person or animal. I am that way when it is a person who died (except my grandmother, then I was very sad for a while), I am ok generally but will be sad or cry when I think of the person, depending how close I was.
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That's the way things come clear. All of a sudden. And then you realize how obvious they've been all along. ~Madeleine L'Engle