Why do most 2 channel amps run hot when bridged?

Why does this happen with *most* two channel amplifiers? I have a Punch 150 run it bridged to a 400 single voice coil sub it's 4-ohms. So I don't know why it gets hot. But this is not the case. I'd like some information on these type of things. Oh yeah it starts to get hot after maybe around 10 - 15 minutes of listening time. I'm not one who blasts it constatly, just to get some bass going on.
The power semiconductors used in amps are derated (can handle less stress) as the temperature increases.

The design of the amp and the type of signal determine which fails first.

It's a Punch 150 (first series) runs kinda hot even at 4-ohms mono I don't know about the gains? But it sounds awesome. Is there anything I can do to keep the temperature down? I don't want it to burn up even in hot sunny days my amp is hot even thought i wasn't using it. Should I install a fan or something?