About the time O'Brien was born.

Plenty of folks become more irascible as they age. It's one thing to do something under orders when you're young, and quite another thing to do them when you're retired.

Also, he would have seen more and more incidents as time went on

Maybe it was kind of like getting B. A. Baracus into a helicopter- they had to drug him first.

Yeah, but once you think it you can't get it out of your head.

I'm going through another cycle of binging EVERYTHING. Yes he did transport regularly, but he also certainly complained about it multiple times. Orders are orders in the end. Sometimes the hardest part of keeping a job is bottling up and repressing all those little existential horrors.

And it was a reliable, proven technology that apparently only got worse and more twitchy a couple of decades later.

I assume the subtext was that it was a reliable proven technology being operated, in his option, by an incompetent idiot too young to be trusted.

Which is as good an explanation as any for the other transporter scene...