I'm with Teethgrinder on this one, though I'll never be able to emulate his superhuman feats!
I did the gym thing for a while, for me it wasn't a lack of motivation on my part or the novelty wearing off, it was the proximity and the behaviour of my fellow human beings. That and the management's failure to do anything about it.
When I wen't to the gym all I wanted to do was get in, crack on, do my workout, shower and go home, but that would have been too easy.
More often than not you couldn't use the machine you needed to because it had been commandeered by a group of three of four males who would deploy the 'do one set, have a pose and a chat then pass to a cohort' technique. By using this method the lat pulldown/bench press machine could be tied up indefinitely.
How about the timeless classic of being stuck between two middle aged women either side of you on the ski machine having a banal, shouted conversation across you about East sodding Enders.........
Never mind, you can always relax in the jacuzzi or sauna after your ordeal... no wait, the gits will ruin that as well. Call me old fashioned and illiberal if you must, but when I'm sat there enjoying the relaxing bubbles I don't welcome some sweaty geezer's meat & two veg dangling inches from my face as they get in because they have chosen to ignore the 'Please wear swimming trunks in the jacuzzi or sauna' rule.
Then there's the repairs to equipment that never happened in a timely fashion. When you are paying a small fortune for gym membership you don't expet to find faulty equipment with an out of order notice for weeks on end, ditto faulty showers, cracked tiles, non-flushing toilets etc.
Compared to all the above going out on your bike and facing multiple assasination attempts by motons seems the height of sybaritic bliss, which is exactly what I'm going to do after breakfast