I was wondering, I flew about 1,100 miles one way then back (no stops). The first path we were very high up I'd assume like 30K feet, the way back we flew I'd say 10K feet, much lower. I was wondering why.
For longer flights, large passenger aircraft will generally aim for something like 30k feet - the thinner air means less air resistance so more fuel efficiency (the cost is the fuel required to get there, which is why I said "for longer flights").
However, at that altitude the "jet stream" can cause winds of 100+ mph. This can be helpful with your direction of travel, or it can slow you down. If the jet stream was strong and trying to blow you backwards, it may have been more efficient to travel at a lower altitude where the jet stream wasn't present.