You have to reseat memory because the contact pins between the memory sticks and your motherboard number into the 100s of connections, and over time with the heating and cooling cycle that goes hand-in-hand with turning the computer on and off, it makes the RAM slightly move.
There are clips that hold the RAM down to the board but those ram sticks still creep a little and reseating them helps.
To do this without damaging anything you need to make contact with your PC's case with your bare hand. Shut the computer off and remove the power cord and then hold the power button down (like you are starting it up). Keep holding down that power switch for about 15 seconds to drain any capacitors that may have any additional charge in them. Now you can safely pop the memory sticks up and then put them right back down. You should get a satisfying click as you put them back and the clips should engage.