Why Clothes Shrink After Wash?
There are many different reasons for this. Most case is because of inexpensive fabric that was stretched when knitted. To reduce the cost of material and make more choice on prices, many inexpensive fabrics are weaved with wider spaces and temporarily fixed, but once it gets into water for long time it goes back to the size what it actually should be. This does not necessarily mean all inexpensive clothing are like this, but unless fabric is weaved super duper tight, the truth is that it will shrink whatsoever just the matter of how much.
It may also depends on the type of materials. Materials like polyester are very weak on the heat, so it is very important not to use hot water with this type of clothing. Even more importantly, it is recommended to follow the care instruction to keep the garment at its best condition as much as possible. Garment is not something last forever nor intangible, so if you found your favorite one and want to enjoy it longer, the care instruction will be your best adviser.
Is That Mean All High Quality Fabric Would Not Shrink?
Just because it shrinks after wash, it does not really mean the fabric is cheap low quality. (I know, confusing right?) Many of high quality fabrics are at its condition for reason to keep its own quality, so,like it was mentioned earlier, if it still shrinks when the fabric is expensive one, it is likely it was not followed by its care instruction. And not all cheap fabrics are supposed to shrink after wash, it's all depends on materials and how it was produced.
What Can Be Done To Figure Out If It's Going To Shrink?
Many of knitted fabrics are likely to shrink when it was weaved stretched. Many of cotton T-shirt or Rayon tops are likely weaved as knitted. Most of those ones that has some stretch itself is considered as knitted fabric. It is not easy to figure out by the human eye for the shirt that's displayed in a store to find out if this was weaved stretch, but if you can find a good comparable garment that is at different price level and looks & feels similar, you can tell by the weight of it. Stretched one, of course, will weigh less than tightly knitted one.
Also, knowing characteristics for each type of different fabric will help to figure out if the fabric is good or cheap.