Imagine too dudes who don't like each other sitting next to each other on the bus. Just like those two dudes, severe storms don't just explode for no reason. they need forcing. Imagine one of the dudes looks at the other the wrong way. at that point, all bets are off and they start fighting. that's the forcing that allows them to start fighting. For storms, that forcing can be the ground heating up unevenly, a front, high shear, and others.