What Causes Lack Of Lubrication In Vagina During Intercourse?

Hi Doctor, Myself 32 years and my wife is 29, we been married for 2 years now. My concern is my wife vagina is too tight for me to enter, but after entering it became loose enough. We do enough foreplay but i could see her vagina is not much lubricating naturally, so we have to depend on oil for penetration, once we use oil it goes in. since I have to use hands and apply pressure on my penis to enter her, in this struggle in normally soften my erection. She is a slim body type. Both of us love and care each other. we need a happy sex life too. Please advice