Why do brassicas like cauliflower or brussels sprouts sometimes taste bitter, and (how) can I avoid it?
All the vegetables form the Brassica family have a bitter component to their flavour. Brassica is the family that includes cauliflower, cabbage, broccoli and Brussels sprouts.

You can actually remove quite a lot of it by leeching it out in to the water, or by using salt. Remember that this means you're also losing some of the nutritional value, though that doesn't matter too much.

You can also try ot mask the bitter taste with the sauce/dressing. Something like a cream sauce, or even barbeque flavour will usually mask the bitterness quite well, while letting the cabbage-ness come through.