Why do we always use square kernels for filters? In image processing, why do we always use odd square kernels(3x3, 5x5, 7x7, etc.) for filters? 

Why can't we use kernels of size 3x1, 5x3, etc. that are rectangular kernels? Also, why do we not prefer even kernels (2x2, 4x4, 6x6, etc.)?
If the kernel is odd then you can center it on top of a pixel, which is nice. Lots of kernels are symmetric, so that's probably why they are often square. You can certainly use non-square or even kernels though.