Ares and Leo are very compatible, because they're both fire signs. So when you put fire with fire, you get a combustion, but in this case it's a good combustion. They love life. They are open to doing, exploring. They love to be passionate, and they love each other.
So essentially this is a good match, except in one area, which is that neither of them wants to take a back seat. Leo needs a lot of attention, needs to feel that he or she is the king or queen of the couple. Ares, in turn, also needs attention, needs to be the adventurer, and needs to be the hero or the heroine. I'm talking about kind of mythological roles that we play in life.
So if the Ares and the Leo can work out who leads, who takes orders, who will make the decisions about money, about finances, about how one lives, about the furniture that you buy, about what you do in bed, they have work out that. Let's say the areas of control. In that case, it will work out beautifully.