Capital gains are created by investing income that has already been taxed. Any tax on capital gains, at least in principle, is double taxation. (This doesn't mean that the optimal tax on capital gains should be zero because in practice, lots of capital gains correlates with lots of competence at generating earned income - so, in a way, that double taxation can be seen as earned-income taxation in disguise. It does mean that there's plenty of reason not to treat them the same.)