For me, the following can help to renew or increase enthusiasm and motivation:

1. Set specific goals, long-term and then short-term. These may include specific events, but need not. They may be purely personal goals.
2. Buy, order, or build a new bike. Short of that, buy some new gear or significant cycling apparel, and have fun testing it or riding in it.
3. Take some time off, but not too much.
4. Join a new club or local ride. Get to know some of the people and their goals and backgrounds.
5. Try a different type of cycling--eg, cross, mountain-biking.
6. Enter a race or other challenging event. Plan and train for it.
7. Take some time to reflect on why you ride and what it means to you.
8. Read a new book on cycling or a cycling figure (Half Man, Half Bike (eddy mercyx), Racing Through the Dark (david millar)) or get a subscription to a new cycling magazine.
9. Experience some major health issues or injuries that prevent you from riding or limit you to very short and slow rides. Not really recommended (I've been there and am still there to some extent) but this really does increase the desire to ride.

Well, that should be a start. Let us know how it goes.