Feelings are so important and so confusing. To a child feelings are strong and overpowering as the emotions center if their brain is so much bigger an stronger then their logic center that any logic they possess has no hope of gaining control for years to come.
This is why tantrums over little stuff (to us) come. Their desire, especially for those under five and even up to seven, are so strong. They can't differentiate between need and want. It is all need.
So what can we do? Our children need to know that they aren't alone. They need to know what they are feeling before they can control that feeling. One way is to reflect their feelings. When a child is crying because of a disappointment, say a canceled play date, sympathies with then. Show them a sad face and cuddle. Say something like, "I see your disappointed. It is sad when play dates are canceled." Then after she has calmed offer an alternative activity to do together.
Also set boundaries, "I see your disappointed, it is ok to feel disappointed, and even angry, but we do not hit people. Hitting hurts. If you are angry and want to hit then hit a pillow."
Or "I see your are really sad. If you need to keep screaming do it in your bedroom with the door shut so that you don't wake the baby."
Feelings can be magnified by physical and environmental influences. Such a need for diaper change/potty trip, a sensory over load (change activity from fast to slow, slow to fast, loud to quiet), too much screen time, tired, hungry, thirsty, needs snuggles and attention.
Remember that this world is a big, overwhelming, scary place. And a child doesn't intrinsically know how to rank things, so he will likely throw an equal fit over food and a toy or trip to the park.
Feelings are good, necessary, overwhelming, and scary. Out children need to be able to identify their feelings, and know that they are normal and controllable. Only we can teach them that.