You started off being a wonderful influence in each other's lives -- offering one another a lot of warmth and optimism and creating a generally positive atmosphere within your relationship. Over time, however, you started taking advantage of the optimism your lover brought to your life. You are beginning to take it for granted, and increase your demands on your lovers good will and support.
Or, you might have an overdeveloped sense of what's possible between you, and have become inordinately disappointed when it turns out that your lover is a mere mortal, capable of negativity and foolishness just like everyone else. A third possibility is that you've become overly dependent on the love and support of your partner, and if you suddenly find yourself without it -- if your lover goes out of town, for example -- you don't know what to do to create that feeling on your own. This easily turns into selfishness; you demand that your lover never leave you alone again, or insist on long phone calls every night, not caring that they cut into both your sleep time. It's wonderful to be able to depend on your partner, but you should both be aware of the negative possibilities that exist between you, and take care to continue standing on your own two feet even as you lean on each other more and more.