Because this:

Holy shit is an erect dick a fucking turn on. It wants. It needs. It yearns. It even literally leaks precum like liquid sexual repressed energy. To me, it’s so obvious why never let him come. But since you’re asking, it means you don’t understand, so let me break it down for you:

The guy on the left was allowed to cum, and now he’s all small, limp, refractory and boring.

The guy on the right has been denied an orgasm, and remains large, erect, horny and fun.

Now which would you rather keep around?

Source: Femdom Planet

I have to ask…why never let him come?