The flu is a contagious infection. Therefore, most people will get it once or twice a year. If you feel that you are getting the flu more often than normal--you should talk to your doctor
as there are some conditions that make you more susceptible to the infection.
Firstly, the "flu" is commonly acquired because there are many different things that can cause the flu. There are numerous viruses that cause the same symptoms, therefore with so many floating around, we are often susceptible to get at least one. Our bodies actually do a good job avoiding a large number of infections, but it will often fail for one or two.
The main reason that the viruses are so contagious is that the viruses are airborne. These viruses aerosolize in the mostly invisible respiratory secretions that we make every time we breath (you can see these droplets when you are in cold weather). The virus survives for a while and then we breath it in. It can even survive is on objects like door handles or hands. In offices there are so many people and there is recirculated air (with heating and air conditioning) that you exposure is so high that it is only a matter of time till one infects you.
Honestly, we don't understand a lot about how viruses are so strong...that is why there is no really good anti-viral medications.
Talk with your doctor if you have more questions. Keep in mind that the flu shot
can reduce your risk substantially.