I disagree with @bbecon. The fundamental argument is that utility experiences diminishing returns.
Examine the image below, taken form here.
Let's say good X is coffee and good Y is cake. If you have 2 cups of coffee, getting a 3rd may increase your utility, but not by as much as the first 2. Idem for 2 slices on cake. But if you could get 1 cup of coffee and 1 slice of cake, you would be most happy.
This argument fails under some special circumstances (e.g. this), but I think it's reasonable in most cases.