While some viruses do jump species, in reality complete jumps are very, very rare. We're all subject to millions of viruses every day, especially those who work or otherwise live with animals, most of these viruses are just not capable of properly infecting humans.
The reason is down to what viruses actually are - to put it in a fairly basic manner, they are a form of life that cannot reproduce on its own and instead has to invade other life (cells) and hijack the mechanisms of these cells in order to reproduce itself - essentially the lowest form of parasite and just like higher forms of parasite, they need to be specialised to do this effectively. From Wikipedia "A virus is a small infectious agent that can replicate only inside the living cells of an organism." (http://en.wikipedia.org/wiki/Virus). To start with, many viruses have to survive in the open environment, or at least outside of a host organism and HIV, for example, is not particularly tough and instead has to be transmitted without using the open environment... which is why, you won't get HIV through (non-intimate or blood sharing) contact with a carrier, through touching what they've touched or inhaling when the carrier has sneezed or coughed. A virus then needs to be lucky enough to find cells that it can utilise, seeking these through the chemical markers of the cells, latching onto these cells, invading them and hijacking the inner mechanisms of the cell. Through all these steps the virus is exposed to being cleaned up, removed, disabled or rendered useless by basic biological and chemical processes or in the case of cross-species, just not able to get far as it will target the wrong genes, mechanisms or chemical signatures. Once in a cell a virus then uses and controls the cell's mechanisms to reproduce itself (some viruses also cause the cells to reproduce, which is the general cause of virus induced cancers). Even once in a cell, if a virus causes the cell to outwardly reflect that the cell is not working correctly then the cell, and the virus in it, will be destroyed by the body's immune system. It's not easy being a new virus!
Viruses mutate a lot, therefore when living continually with animals that are shedding viruses there is always a chance that one virus strain might happen to be able to infect a human. A certain level of "infection" is quite common but usually entirely harmless as the viruses fail at some step of their hijack and reproduction process. For example, the virus might be able to infect a cell but not to reproduce. Even should a virus happen to manage to reproduce itself, it still has to find its way from one human to another, and that's a very different problem as that involves getting out, crossing between hosts somehow (HIV takes shortcuts on this and needs direct transmission) and then evading the immune system and defences of the new potential host.