When you idealize something, you think of it as being much better than it really is. You might idealize your beach vacation, completely forgetting that it rained almost every day.
If you idealize your favorite teacher, you imagine that he's even more wonderful than he actually is. Many people tend to idealize marriage, believing that simply being married will make them and their partners blissfully happy all the time. To idealize is to take an ordinary, flawed thing, and turn it into something ideal. In fact, ideal, or "perfect," is at the heart of idealize, with its root of the Latin idealis, or "existing in idea."