…there are two kinds of tantrums and one often follows from the other…
The first kind of ‘tantrum’ comes about when a child is between two and three years of age as a result of not being understood by the adult who is closest to the child.
You are the boss and the leader in the eyes of your child. If you grovel, apologize or say "how sad this makes you" when he or she throws a tantrum when you have stopped him from doing something that may be potentially dangerous, you tell the child that HE IS THE BOSS!
Not only do you give your power away, but you load him up with a responsibility he is not ready for.
A young child does not understand these dynamics, it doesn't make them feel powerful, but insecure. This will be expressed by the child acting like a little dictator and pushing boundaries, not because he feels strong, but because he doesn't know what to do with the power he now feels he has.
Take your power back to where it belongs: with you. You are the parent, let your no be no and your yes be yes. Set a clear rule of what is permissible and if the pre-schooler does not stick to the rule, simply tell him that he has to grow a bit more before he is given another chance.