Why does my hair get so greasy?

I have to wash it every day or it gets so greasy and disgusting it didn't use to be like this, what has happened? I am male, 31. If i don't wash it everyday i look like a tramp and even get security guards following me round in supermarkets thinking i am going to steal something as i obviuolsy look like a poor, or dodgy, character, very annoying!!