Why does my pictures look alot different on my iphone than on my computer?

When I take a photo of myself it looks a lot better on my iphone than on my computer. Why is this? I think it has something to do with the brightness on my iphone and computer but I don't know. How do I fix this problem?

Added (1). And my photo looks different on different computers. Like when I use my computer my photo will look completely different from when I use someone elses computer

Added (2). Its definitely the brightness but how do I fix the problem

You'd have to be more articulate to really diagnose the problem.
But it sounds like maybe the image looks better, smaller, because the limited pixel range masks some of flaws in the photo.

It's like when you click on an avatar, and then see the real photo and you're like, oh, she looks better as a thumbnail.

Same reason people aren;t ugly from across the street, heh.

I think it's normal because the screens are different
iPhone's is small and computer is bigger
you can clearly see all un-beautiful corner

IPhone is already has the Retina Screen，you probably change a big screen like Samsung Note2

The computer is showing you nearer the reality. You need to calibrate your computer monitor or screen, you can't calibrate your iphone.

Once calibrated your computer will show you accurately the colours and the brightness recorded by your camera, any camera.

You need a ColorMunki, Eye One or Spyder to calibrate your monitor, the ColorMunki can calibrate your printer too.

If all the computers you have access to are calibrated too, then your images will look exactly the same on all of them.

On your iphone the image is tiny - you can't see the detail properly - it only looks OK because you can't really see it well.

On your computer, it's much much bigger, and you can see all the flaws, and the general lack of contrast which is common with typical poor quality phone cameras will be evident. All camera phones are poor quality. What you see on the computer screen is much closer to what it really looks like.

Phones are no good for photography. Get a PROPER CAMERA.

Photos look different on different devices, this is a simple fact of life - there's nothing to fix. You can't calibrate everyone's computer monitors so that they all display exactly the same. It's NOT possible. Heck, photos will even look different depending on what angle the monitor is positioned at.

The suggestion to buy monitor calibration hardware for your computer to display pictures from your iphone is quite frankly ridiculous. Instead, use the money you will save to buy a cheap point and shoot camera, which will be 1000 times better than any phone camera.