Supporting Ukraine's right to self-determination sounds like a good reason to support them - but I'm pretty sure that's not why we are doing that...

Russia is a nuclear power. NATO is also nuclear (US, FR, UK). NATO military very much overmatches RF. If there were to be a direct confrontation it would be one-sided, probably to the point of the RF using nukes out of desperation - at which point escalation to a global exchange becomes a real risk — so the RF needs to be stopped before it can get to a NATO border - probably Poland, even though Putin has been sending signals (Northern War talk) about the Baltics.

Russia is just doing what it’s done historically: expand to the point that it has easily defensible borders. They need to do it now as they’re in the middle of a demographic collapse; there won’t be enough men of conscriptable age in ten years. As it is, they’re drawing primarily from non-Russian ethnicities.

All that said, it’s entirely possible that Germany walks away from NATO over Russian energy — which could lead to the US leaving NATO.