Why does a dominant allele always increase in frequency?

Because as the dominant allele is passed down it becomes even more common - maybe

The dominant allele increases in frequency as it masks recessive alleles. Not all individuals have homozygous dominant alleles but even with a heterozygous combination of alleles, the dominant allele/trait will mask the recessive allele/trait. That is why a dominant trait/allele will become something is more commonly passed down to offspring.