Why do I feel jealousy?

Okay, I am 19 and I don't plan on having any babies soon. I don't want a baby but as soon as I see someone have a baby or find out they're pregnant I feel instant jealousy.. What is that? My sister just had a baby and when I went up to the hospital to see my nephew I just felt like "well where's MY baby??" 
Like it just doesn't make sense. It's a completely illogical feeling. Is it natural to feel this way? Like I'm happy for everyone I really am like everyone around me can have all the babies they want I get my baby fix and I'm good.. I just don't understand why I feel this way. 

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