Kids cry for all kinds of reasons: injury, attention, jealousy, more attention, and just because.
Some kids use crying as a form of language, for instance they might want a glass of water, “waah nnaaer whaa!”
Unfortunately as a parent it is your job to discover what they mean or want from this crying language.
Sometimes you just have to close your eyes and channel some mystic God to give you the answer.
Other times you just need to move around and point at things hoping that one of them is something that they want.