Those times that I have no enthusiasm for cycling precede an actual ride and are usually about one of two things....it's early season and everything is "too hard" or I'm putting pressure on myself to go fast. I have found that when I go out with my gf I have less reluctance to ride. I also find that riding the trails with my cross bike does not have me feeling reluctant to ride. I think this is because the rail/trails are relatively flat and I have no need to go fast. I don't know if this bears any similarity to what you experience but perhaps? I have noticed that once I'm actually on a ride everything is fine.