He inherits his Daddy's yawn.
Just like his Daddy has done for a long time, Seth just started using his vocal chords to yawn. Kids have all kinds of quirks that come out in different phases, talking loudly while breathing in; that's a lovely one, especially right in your ear. Yawning with a voiceover isn't so bad. Even when he (1) does it in public. There are far more disruptive things a five year old boy could do.
I don't imagine Seth ever had a moment where he thought, "Hey, I like the noise Daddy makes when he yawns. I'll start that." But it's at this age that boys have really begun to identify with their fathers. (The process of pulling away from the mother can start at about 18 months.) I think even this goofy, subconscious adopting of a trait seals the deal. We are destined to become our same sex parent, no?
Naturally, there are little troubles and stress (2) that you learn to let go of with each child. It may be the best reason to have three children. As Daddy has yawned loudly through our years together, I've learned to transpose annoyances into amusements. And there is the secret to a happy marriage!
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1) Seth, not Daddy
2) As little as they are, they step on my feet and I still holler "Feeet."