When you ride a lot, the battery will maintain itself. Not fully, because your battery wears out over time. And at some point you still need to replace it. But a motorcycle battery that sits idle slowly discharges. This does not happen overnight, but it definitely will when it stands still for several weeks or months.
The cold during wintertime accelerates this process. So no happy battery. The cold increases the resistance in the battery, reducing its capacity. Do you keep your motorcycle in your living room in wintertime? Then that helps your battery a bit, but even then the battery will slowly run out. That’s why motorcycle battery maintenance in wintertime is important.