Partly, I'll admit, it's because it's just karma. You wake me three times a night for two years, now it's MY turn, so there. How do ya like them apples?
But more, it's just such unmitigated sweetness. There's that brief moment of time, in between asleep and awake, when they're moving enough to snuggle in and they're still warm and smell sweet and have the sleep-warm pink cheeks that I just cannot kiss enough, but they're not conscious enough to require anything more than simple snuggling. No requests for food, TV or toys, no chattering or babbling or muttering. Just snuggles.
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[Edited to add: I just put a bunch of flood pictures on my photos site for those who are interested...