There are several tough problems to solve before years-long deep space missions can be carried out. For starters, we have to be able to build spacecraft capable of insulating astronauts from cosmic radiation. We have to deal with either the biological consequences of extended weightlessness or provide some kind of artificial gravity (a severe problem for a ship of limited size). We have to have closed-loop life support systems that can operate for years without failure. We have to ensure that astronauts can psychologically handle being trapped in a small space for the better part of a decade. These problems may not seem all that big to you (I can already hear "just add shields," "make the ship rotate," "have the astronauts psychologically qualified," etc.) but the solutions are not so easy when you get into the details. Look at how many technical problems arise on the ISS, which is large, in (relatively) easy re-supply range, and in a much less harsh environment. At least on the ISS, you can drop home on a Soyuz if things go too far south. Not so on a deep space mission.