Squats seriously get me high. Any hard set on legs gives me a super ridiculous head rush, I don't think it's from the breathing either because it generally lasts far through until the next night in terms of mood and libido. I feel like a hard leg day always gives me awesome sleep that night too. Here lately, the main motivation for busting ass on my hard leg day, which is Sunday, is
a. awesome sleep
b. awesome mood lift
c. sets up a good beginning of the week. I hate mondays, but if you kill your legs on sunday, a shitty monday is better.