It's mainly because laptop is very compactly designed and the machine itself can't drive the heat away from itself.
Too much heat shortens the lifespan of the components.
- Lift your computer up - if you use your laptop as a desktop ( on a table ), you can use some helping tools to lift up your computer a bit. Tools like ( you need 4 pieces of these ) mini napkin packages, matchboxes, or any other relatively small items. You just have to place them on the corners of the laptop and that makes some room for air flow. If you don't want to use amateur tools, you could just buy a cooling pad, which is specially made for better laptop cooling. Also, books can be used in this matter ( 1 in each side of the laptop ).
- Clean out the dust - Dust is everywhere and it also tends to group into the laptop heat sink. Dust is not a good heat conductor and it keeps the heat locked in your computer. You have many ways how to get it out of there.
1) Clean it yourself - For a good clean you need a good air pressure, best and easiest way is to use a compressor. But you can also use something else, might even help if you used your own mouth. You need to blow into the heat sink ( be careful if you're using compressed air ) and if you have dust clogged up in there, you'll see it coming out as you send the air in there.
2) Send your laptop to service - Some companys do it for free if you have a valid guaranty, others charge you about 5-10 $ for it.
I recommend you do this atleast 2 times a year.
- Keep the airholes cleared - If you hold your laptop on your lap, try to keep the heat sink hole and also other air-holes cleared. Position the computer so that you don't cover the holes with your legs.
- Keep away from hot places - Try not to use your computer in places where there is high temperature ( fireplaces, ovens, etc ).
If none of these steps above help you, you might have a broken cooling system. ( check it by placing your hand in front of the heat sink and see if there is any air coming out of it, or if you just can't hear the cooler make the sound it used to )