How do babies learn to fall asleep by themself?

Sa

WITHOUT using cry it out. How does a baby learn to go to sleep themself?

I can barely get my daughter to sleep holding her and rocking her and singing to her etc. I do not ever want to use cry it out so I'm wondering what the alternative is?

My dad insists I'll "have to do it eventually" because he doesnt believe there is another way... but there must be! Right??

She is only 4 months now and I'm not trying to force it anytime soon... but just wondering for the future.