why is like this and how to fix it?
You need a seamless, tileable texture - see Arroway Textures for examples.
As RI3DVIZ mentioned you need to use a seamless texture to be able to tile it without seems. If your issue is the frequency of the tiling you either alter the UV coordinates of your mesh or alternatively you can use a Texture Coordinates node in the material editor and connect it to the UV pin of your Texture Samples. For instance, using a value of 0.5 will make your texture tile half as much, using a value of 2 will tile it twice.
I hope this helps you a bit.