Change of wheel: Can’t you get the wheels down?!

Hello, I have a curious problem: when changing tyres in summer -> Winter tyres I solved all the wheel nuts. But I still didn’t get the tire off! It was like baking it! Everything pulled and (one side) didn’t help. Did someone give me good advice on how to get on with it? Is there something like lubricant that I can use to work with the rim/axis? In general: how can a rim make itself so tight? Thank you! koasa