There is a leak in the bathroom and its coming through the ceiling in the kitchen, why is this happening?

Usually Id say its possibly caused by a shower above if the sealant around the base is letting water pass, it will need the old sealant removing & re sealing with a bathroom sealant available from most diy stores - otherwise it depends what is directly above the leak

- otherwise it depends what is directly above the leak Not always true. Water, like electricity , will take the path of least resistance. Unless you can spot a leak upstairs you'll have to open up the ceiling and track the leak back to the source.
Let me know what you find. Good luck, Tom