I don't regret anything in my life prior to the conception of my sons. This may sound like sentimental nonsense, but I tell you it's true. Here's my argument:
1. Basic biology: A man produces hundreds of millions of sperm every day. Each of these sperm contains (half of) the genetic blueprint for a different person. The slightest physical movement changes the position of sperm.
2. Therefore, any change in my life prior to my children's conception would have led my children not to exist. If I had crossed my legs differently, or walked to the frig, or even chuckled an extra time, the sperm would have been rearranged, negating my children's existence. I might have had different children, of course, but they wouldn't be the ones I have.
3. Like most parents, I have a massive endowment effect vis-a-vis my children. I love them greatly simply because they exist and they're mine. If you offered to replace one of my sons with another biological child who was better in every objective way, I'd definitely refuse.
4. Therefore, if you offered me a "do-over" on any aspect of my life prior to my children's conception, I would refuse, for it would mean that these specific children would never have been born.
5. Since I wouldn't want to change any event prior to my children's conception, I have nothing to regret. And since I have nothing to regret during this period, I don't regret anything.
Note that this argument only rules out regret prior to the conception of your last child. Events between the final conception and the present wouldn't change the identity of your children, so the argument from the endowment effect doesn't apply. It also doesn't apply if you wish you'd had a different child, or no child at all. But your child has to be pretty rotten to warrant such a wish, no?
If you think this is just my egghead way of saying "Happy Father's Day!," you're only seeing the tip of the iceberg. What I'm really saying is that if you love your children just because they're the ones you got, you have a special reason to be happy every day. After all, you can survey your whole life before your last child's conception and honestly say: "It all happened for a reason. I wouldn't change a thing."
P.S. Happy Father's Day - and if you're reading this, Dad, thanks!