My DD is 6 weeks old and sleeps like an absolute log next to me, co sleeping. We've given her crib a few tries and she hated it
I've read online that it's to do with their breastfeeding being on tap but she doesn't ever feed during the night, so long as she's next to me
If she's in her crib, she wakes to be fed a few times
DS had the exact same pattern of behaviour at this age too. He slept beautifully and I was never sleep deprived, but he was in my bed too 
Why do they like it? I don't particularly enjoy the presence of my own ex H... so can't say in the same and hate sleeping alone
I like sleeping with my DC but prefer my own space generally speaking