This is because he can see a mare and foal across the field (he's got her in foal anyway) and can't actually touch her.
As it's so dry here there were clouds of dust which really made him cross.
He then turned to knocking over any jumps in the field with his nose!
At the moment he is on three conditioning feeds a day plus lots of grass and haylage when he's in, but sexual frustration takes weight off a chap as you can see by his ribs..
The good side is he's very fit... he easily makes the time XC and barely sweats.