It's perfectly normal to want another bike. Need is a non-factor, unless your son or daughter takes a sudden interest in cycling and steals your only bike away from you. In that case, it's imperative to fill the void as fast as you possibly can before the absence of a bike forces you to go running, or worse. Likely though, you have a bike or two or more. There was a time when each of those bikes was all you ever wanted, but what are you to do when something new catches your attention? Go out and buy it, of course.
Law #1 - The force of N+1 is always at play.
Approximately one second after getting a new bike, the imaginary N+1 timer resets. It's not your fault. Enjoy riding and admiring your new bike! There's nothing quite like #newbikeday.
Law #2 - The strength of N+1 force is both progressive and regressive. N+1 is absolute.
A trip to your local bike shop increases the attraction for a new bike, as does seeing your riding buddies posting photos of their new bike on social media. Unexpected car repairs or pressure from a significant other reduce attraction for a new bike. Baseline is always slightly positive from neutral and is the regression limit point.
Law #3 - The time span of N+1 from one bike to another is infinitely variable.
It's less fun when forced, and you shouldn't take it too seriously. N+1 happens when it feels right. Bikes are more enjoyable when they are being ridden, so continue putting miles on your current bike(s). There's nothing wrong with them!
There is at least one good reason for getting a new bike, in addition to the many health benefits of riding it. Whether as an upgrade, an addition to the weekly rotation, or an extra motivational push, the joy of a new bike is a fact of life as a cyclist. Ultimately, the decision to act on N+1 is up to you. The concept of "one more" isn't something new to cycling. One more mile, one more hill, one more loop. One more is always just enough. How is one more bike any different?