Why is my water heater leaking?
It could be a loose fitting or the overpressure valve kicking in, but not likely.

The tanks in most water heaters eventually corrode, how fast depends on a number of factors, one of which is the amount and type of minerals in the water, as well as its acidity.

Once they do leak, repair is generally impractical and a new water heater is needed.

In any event, you need a plumber to check it out and solve the problem.