When you drain pasta please do not rinse the noodles! You are washing off many of the nutrients and the starchiness that allows the sauce to stick!
If you refrain from rinsing you can instead coat the pasta with a thin layer of sauce. For example, if you're fixing spaghetti, drain the noodles, return them to the pot and add a few tablespoons of the marinara sauce, stirring well to combine. This will keep the noodles from sticking to each other but will still allow the sauce to stick to the noodles.