1. You need to get pumped up.
2. You are in a bad mood.
3. After you break up with a girl/boy
4. To relax
1. Probably Iron Maiden or Cream
Examples can be but are not limited to The Trooper [http://listen.grooveshark.com/#/s/The+Trooper/20YezW], Childhoods End [http://listen.grooveshark.com/#/s/Childhood+s+End/PhaNG], Brave New World [http://listen.grooveshark.com/#/s/Brave+New+World/2Aqsk], Sunshine of Your Love [http://listen.grooveshark.com/#/s/Sunshine+of+Your+Love/2Gao07], and White Room [http://listen.grooveshark.com/#/s/White+Room/23Vmlv].
2. Pitchshifter
Something like W.Y.S.I.W.Y.G [http://listen.grooveshark.com/#/s/W+Y+S+I+W+Y+G+/23Pt3v] or Genius [http://listen.grooveshark.com/#/s/Genius/2gGO8E].
3. Element Eighty, maybe Deep Purple.
Broken Promises [http://listen.grooveshark.com/#/s/Broken+Promises/3I8eM]...
4. All of the above plus much more...
AC/DC, Motorhead, R U S H, KMFDM, etc...
I strongly reccommend a click on any of those links...