Aries and Sagittarius are a wonderful, combustible mix. Both have fire signs and fire in astrology stands for passion, excitability, adventurism, and an ability to be very open to life, and that's what both share and have in common.
The combination works well when each of them has an area that he or she controls, because neither one wants to take orders from the other. Sagittarius being the sign of freedom and non-restrictiveness certainly doesn't want to be bound into another person's rules.
Aries on the other hand, wants to be the one that calls the shots, that is the leader, that is the first to say, 'Ah-ha. We need to do this. We must do that.'
So if they can work out that they are good to go. They enjoy life. There's a sense of optimism, a sense that life is there to be sampled, like a feast. They both come to the table ready to feast on what life can give to them. They both are explorers, they love new adventure.
They enjoy other people. They have a great deal in common, including a sexual nature which is of enjoyment, of combustibility, of being passionate people. And what they need to work out is who is giving who the orders.