Regardless of how enthusiastic, or excited you are, you will eventually hit what I like to refer to as "the wall". This is where either you don't care about the hobby anymore, or any maintenance, or spectating of the tank is no longer out of enjoyment, but out of habit.
I myself have hit this wall a couple times myself. I am now able to recognize the signs ahead of time, and find ways to prevent it.
You might have hit the wall if: You put off water changes for no reason, you don't do basic maintenance such as temperature check, you no longer find interest in looking at your tank, or if you lose desire to learn more about the hobby.
For me, the most recognizable sign is the latter. I am an extremely avid reader and poster on numerous Reefing forums. At my peak, I have 4 or 5 that I visit numerous times per day. But when I have lost interest, it usually shrinks to 1 or 2. This may seem like a trivial subject to talk about, but I find it very important, as any loss of my interest can have detrimental effects on my tanks.
What might cause this: One large loss, such as a favorite fish, tank crash, maintenance redundency, not enough time to devote, etc.
Being a full time student, and a part time sandwich maker, I normally have very little time to devote to my tanks on a daily basis, and rely on weekends to take care of water changes, and other tasks. I am not home from 7 am - 9:30 pm usually, and by the time I get home, the light on my tank is either shut off, or about to shut off.
Now, most importantly, what can we do about this? For every person it will be different. For me, its the excitement of planning a new tank. For example, about a month ago, I hit the wall, bad. I was considering selling all my tanks, and quiting. One fateful trip to the LFS got me a 65 gallon for a bargain, and that was all it took. Since then, I have enjoyed a healthy tank and renewed interest in the hobby.
Perhaps this doesnt affect everyone else such that it affects me, but I can guarentee that it does happen to everyone eventually. Learning what might cause it, and realizing what you can do to retain intrest might help you to save your intrest.