You aren't terribly realistic when it comes to your expectations in love. You might think you're just optimistic, and in one sense, you're right -- it's great to avoid the trap of cynicism that so many people fall into when it comes to affairs of the heart. But you have an idealistic view of love itself and of your actual romantic partners that can lead to a lot of problems.
You expect perfection, and you can delude yourself for quite a while that you've found it. You'll look at your sweetie with stars in your eyes, blind to all their faults -- at least, for a while. You can convince yourself and everyone around you that you're blissfully happy, which creates a steep slope to slide down once reality intrudes. While it's nice to hold on to your ideals when you're involved romantically with someone special, it's also essential to maintain a sense of reality. They aren't perfect, and nor should they be. Perfection in a human is impossible, and it's an unfair standard to hold someone to -- especially someone you care about. But beyond that, you'll keep being disappointed in love until you can learn to view your sweetie holistically and accept their flaws along with their wonderful qualities.