Why does my puppy insist on sleeping in my bed?

i have a mastiff american bulldog mix puppy he is 14 weeks and we have never put him in our bed he just crys and jumps up for agesss ( up to 40 mins) untill we let him up, when he falls asleep which doesnt take long we put him back on the floor and he sleeps all night unless he does wake up and realises hes on the floor he crys again to get back up. this is only at night as during the day he is more than happy to sleep in his bed. i no we probably shouldnt give in but he gets really distressed. i would like to know why he does this and maybe how to gradually stop it?...