Crying is always seen as a sign of weakness, and personally I think that's ridiculous.
And I think the entire idea that crying is only for women and that a man shouldn't cry is even absurd.
We all have the moments that life can be extremely cruel on us, even to a level that we simply cannot take it anymore. Crying somehow let us blow off the steam that came to a too high level of pressure. And you know what happens to a steam engine when the kettle is under too high pressure... Yup, it'll eventually blow off, hence the driver being required to vent the steam to lower the pressure. If you take your mind as the kettle and your emotions as the pressure, and crying as the vent... well, there you go.
And I've never been ashamed to admit that whenever the going gets too tough on me, I'll allow my tears to come. However as I have been raised under the old-fashioned idea that a man shouldn't cry I am sometimes unable to let my tears flow, and really I always hate it when that happens.
Not understanding the reasons why you cry can only mean that there are some bad things in your mind that you are not fully aware of. Maybe you pushed them away somehow (as society wants us to do that). They key often lies in the past. Sometimes some memories of decades ago can make you cry now while you couldn't do so back then. It's quite strange.
If it really goes on an on and on, and you keep failing to find out why, you may need to talk to a psychologist.