This has been a huge problem for our group going back a long way. The culprit? Generally irregular play! If we are playing with a regular group we usually stick together, get excited and cannot wait to play again. If the characters are not played for a few months, everyone forgets what made the group so great and when the urge to play hits, someone suggests something new on the fly.
I hate this problem. Of late, I have endeavored to really be consistent and even DM more to get it going. I want a cohesive party.
One other problem--a much bigger issue post AD&D 1e (i.e. 3rd edition on) is a focus on "builds" and ability combinations. Longevity for me is when I am immersed in a theme and the character.
I recently started a thread about the tension I feel in making a "better" character vs. a "better character." When I look through a lens of optimization only, the character is more disposable, less exciting. When I take some things that follow the rule of cool first, the character usually has longevity.
Just my 2 cents. Immersion leads to less switching for me. I have an internal struggle to take more flavor along with optimal things. When I do, I am interested much longer.
My thought is that perhaps this is a function of novelty wearing off. The exciting mechanics are only exciting for me in the planning and early stages. Later, they don't excite me so much. Theme however, really keeps me invested. For example, if I can mesh background and class with some spell choices and even tool use? Jackpot!
