You start 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, your lover might start taking advantage of the optimism you bring to their life. They might begin to take it for granted, and increase their demands on your good will and support.
Or, they might get an overdeveloped sense of what's possible between you, and then be inordinately disappointed when it turns out that you're a mere mortal, capable of negativity and foolishness just like everyone else. A third possibility is that your lover becomes overly dependent on the love and support you give to them, and if they suddenly find themselves without it -- if you go out of town, for example -- they don't know what to do to create that feeling on their own. This could easily turn into selfishness; they could demand that you never leave them alone again, or insist on long phone calls every night, not caring that they're cutting into your sleep time. It's wonderful for them to be able to depend on you, 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 as time goes by.