because you haven't crate trained him to know that he must be quiet in his crate. He's trying to test you by crying to be let out, if you have given in even once then he's learned that this works. Remember, when you take him out of his crate he must be quiet when you're doing it. If he was barking or crying when you let him out then he's gotten it into his head that making noise will get him released.
If you've made sure that he associates his crate with nothing but POSITIVE things (which means you've never punished him and put him in the crate) then you just need to reinforce the positiveness. For example only give him a certain toy when he goes in his crate, give him a kong filled with treats and peanut butter to keep him occupied for a few hours.
You didn't mention how old your dog was, if he's a puppy then he's still being crate trained. If he's an older dog then he might not be crate trained properly or has never been in a crate. It takes time, positive reinforcement and rewards for only when he's in the crate.
Ignore crying this behavior will stop if you ignore. Depending on age, if he's a puppy then the above is enough activity right now, if he's 6 months and up then the above is not enough exercise you'll need to take him on 30 minute walk 2-3 times a day to help him release any frustration he has. You should reduce his meals to 2 x per day if he's older than 6 months. Smaller portions two times per day is usually better than one large portion.
Dogs do get bored, and if not sufficiently exercised then they will find ways to release that frustration, it will either be barking, chewing, and aggression.
First make sure you've crate trained properly and give him things to keep him occupied like a Kong toy. Sooner or later he'll learn that the crate is his "safe place" which means never using the crate as punishment. If sufficiently exercised your dog should have no problems going into his crate, make sure you exercise before crating and immediately after. Place a shirt that you've worn inside the crate so he has your smell with him it's comforting.
You're the pack leader, which means you're the boss not your dog, what's cute as a puppy IS NOT cute when they are a full grown dog, remember that the next time you laugh at his bad behavior because it's cute, imagine how not cute it would be if he was full gown. Also, if you own a small dog, don't let him get away with bad behavior because he's small, he will still need rules, boundaries and discipline, small dogs develop very bad aggression problems if not corrected young.
Owner of a Weimaraner