What is a happy horse?

My horse is 21 and still very high energy and athletic. He’s sharp as a tack and ornery. I’ve always felt shame that his potential has been “wasted”. But now I’m wondering - isn’t this an anthropomorphic projection? Is my horse not happy being retired? Is he really that much happier with being worked in the round pen for an hour and then being turned out? Why would I stall a high energy horse from 10am-7pm and why is that better for him just because he is high energy (a trainer suggested this)? Would his quality of life really improve if he was stalled and ridden daily? He is in perfect health. He’s attentive, observant, and has personal agendas. He’s gone without consistent working or riding for 9 years. Admittedly he does seem vaguely annoyed all the time with his pasture mates, and can be a little antsy for certain people when being held with a lead.

When you think of a happy horse - what do you see in your head? How do I be sure my horse fits into that category?