Why electron density increase on ortho para positon?

I am assuming that you are refering to a benzene ring, with some sort of subsituent group attached?

If this is the case, the electron density of a benzene ring can be altered by attaching different substiuent groups to it. A group can either push electron density into the ring or withdraw it. If electron density is pushed into the ring from the substituent, either along a sigma bond or through the pi system, then the density is concentrated at the orthro and para positions due to the resonance structures formed when attacked by an electrophile at either of these postions.

Very difficult to explain properly without pictures, I am sorry.

Wikipedia has a page, with more detail under the Ortho/para director heading:

http://en.wikipedia.org/wiki/Electrophilic_aromatic_substitution