I agree with suizenji.
It has been frustrating for me too.
I'm working with a graphics tablet, doing illustrations. I use hotkey 'i' for eyedropper, click somewhere on my image to get a color, then immediately 'b' for brush. I would expect to be painting now with the color I just selected, except that I'm not. I now have to press 'x' to change to the color I want.
...
I've discovered that it's not such a big deal, as it seems I only have to press 'x' once and then 'i' and 'b' do what I expect them to. (After assigning 'b' in preferences to only use the one brush) It was frustrating me for a while, but I realize now that I'm just learning the quirks of new software.