port-forwarding - why?

Hi all. Quick question: why is it that when surfing the net, my router knows where to forward my data (ie, to my computer), but when running a g**e server or FTP server, or using BitTorrent, it DOESN'T, and I have to forward my ports?

I know this must be a deep technical issue that gets into the coding aspects of TCP/IP or routers, but it drives me nuts.

Is there some app that will cure this for me? Look how many ports I have to maintain: Sad ...and whenever my network IP gets changed (often), I have to get in there, and fix all the numbers. (example: this morning, my network decided that my computer should go from .102 to .104... Yay.)
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