Very, very simple. It is because you are not healthy, your immune system is weak. Do a search on how to build up your immune system and you will find basic, easy tips on how to get yourself healthy. When you are healthy, you have provided your body the tools it needs to fight of sickness. If you are truly healthy you will not get sick.
some ideas on how to best boost you immune system are; eat more green vegetables as this helps white blood cells.
dark fruits are high in antioxidants and these give you a real boost. Try drinking purple grape juice in the mornings or have some blue berries with your cereal. Another idea is to take a supplement during the cold months when colds are abundant and fruitts are out of season. Something like a black elderberry liquid supplement would give you that important immune system boost.
Vitamin C is great too and you can get this from orange juice or also in a supplement like a dissolvable tablet.
Try to keep yourself hydrated and rested. Hopefully you wont catch anymore colds this year.