Babies are alive because they're alive. They do not choose to remain so. They do not choose suicide because they don't know what suicide is. They don't know what "being alive" is, either. Babies are not capable of choosing between complex ideas such as "life" and "death". They... are... BABIES. Their little brains are not developed enough to comprehend the things you're claiming they're capable of.
If a newborn dies from SIDS (crib death), do you think they chose to die? If not, why do you assume that because they didn't die from SIDS last night that they chose to remain alive?