There are actually a few different mechanisms for a trait to be dominant vs recessive.

When an allele causes loss of function of the protein it makes, that’s often a sign that it will be recessive. For example, brown eyes are dominant to blue eyes, because the allele for brown eyes makes a brown pigment in the iris of your eye, while the blue allele simply doesn’t make the brown pigment (there isn’t a blue pigment, blue eye colour comes from the structure of the eye and the blood vessels running through it, like the veins of a pale skinned person). In this case, the recessive blue phenotype is simply an absence of pigment, so it can easily be masked by one copy of the brown allele, making the pigment. You’d need two blue alleles to have blue eyes, but one brown allele will do.

Other times, it’s a bit more complicated. For some genes, “dose” or number of copies is important, making an allele that has a product effectively recessive to the loss-of-function mutation because not enough of the product can be made from only one copy. There are also cases of codominance and incomplete dominance.