Try to see it from the other point of view, too, of the person trying to pull out onto the road you're on.
Every morning on the way to work I have to cross over 2 busy roads to get the way I need to go. To not cross these roads would require me to take a 4 mile de-tour.
Most days, the only way I can get across and turn right is to pull out into the first available gap in traffic approaching from the right, then, sit there and hope that someone coming from the left will let me out. If I wait for a gap both ways in the traffic before pulling out, I will never get out.
There's always traffic at the roundabouts at either end of the roads, so whether you slow down a touch to let me out or not, you're still going to have to stop at the roundabout at the end.
The same thing happens in the town centre when I'm trying to come out of a parking space on the high street. As I'm crawling out, people will actually swerve round me, sometimes half way onto the other side of the road, before they will let me out. I've been sat there for well over 2 minutes in the past, creeping more and more out into the main road, until practically my whole car was out on the road, and people were still going round me! Slow down and let me pull out! They even do it when the traffic lights that are 50ft further down the road are on red - they'll swerve round me and then stop directly behind me, again preventing me from pulling out!
It's not always the fault of the person who's trying to pull out - it is also quite often the fault of the person on the main road, too!