It's a heart thing, because you suddenly realize you love someone more than you've loved anyone else.
It's a fear thing, because you're more vulnerable to your child than to anyone else.
It's a guilt thing, because parents are only human but that doesn't make you feel any better.
It's fatherhood, and the feelings are felt by biological fathers, stepfathers and even single mothers who have to also function as fathers.
"I was very nervous when I first became a father," Imperial resident Gerardo Antonio said.
The 29-year-old was playing with his daughters, Cami, 10, and Bianca, 3, at Bucklin Park in El Centro on Friday evening.