It is weird to see two different mythologies so closely related to each other. Roman mythology seems to have some Greek mythology and vice versa. For example:

Greek - Zeus, Roman - Jupiter

Two of the same gods with different names. Much of the two mythologies are closely related to each other and it can be confusing to know which is which. Why are the two mythologies so closely related to each other and what is the major thing that makes them two different mythologies?