Resisting sleep

Sa

Why do babies resist sleep so hard? This has to be the single most frustrating thing about parenting up to this point. My LO is 4 months old and fights naps and bedtime with everything she's got. It's like wrestling a tiny angry alligator. She can be peacefully asleep on the boob for 40+ mins but as soon as she pops off and DH picks her up to hold her upright for awhile before laying in crib she is wide awake and wants to party. I just don't get it.