Zeus admired mortals because of their ability to learn and grow unlike the gods who had set personality traits. Nag all you want Hera, Zeus will be a horny dude. He can’t help himself.
One day Zeus was looking down on a group of mortals girls picking flowers. One girl (Europa) caught his fancy. Zeus descended down to meet with them disguised as a white bull. The white bull was a good choice because it was unusual but not too out there. The girls may have been afraid of an elephant for example.
Anyway Zeus the Bull was acting all tame and stuff letting the girls pet him. He lay stretched out on the ground and Europa climbed on top of the bull’s back.
Seeing his chance Zeus ran into the ocean with Europa still on his back and swam all the way to Crete. By that time I imagine Europe would have stopped screaming in terror. Once on shore in Crete Zeus changed back into himself seduced Europa and she ended up giving him 3 sons before Zeus lost interest and moved on to other conquests.
Meanwhile, back on the mainland Europa’s family was worried sick and her two brothers set out to find her. They never did, but that is a different story.