Stooping to pick something up from off the floor is an example of a common movement pattern practiced. This movement places a lot of stress on the knees (standing-stoop & stoop-stand). This stress we may interpret as pain. Over time, this movement, as well as others like it, wears down the supportive tissues of the knee(s) such as the meniscus and ligaments, among others. Learning (re-learning for others) and practicing knee-saving movement patterns to stand from the floor are important towards you experiencing a pain-reduced/pain-free life.
Watch. Listen. Practice.
