Someone said that buying a lighter bike leads to delusions, which is a mental illness. I think that the cause and effect is sometimes reversed from that. Delusional thinking leading to the purchase.

Reality is physics, amateur or otherwise. Ten pounds less from 180 pounds total is 10/180 less effort raising that weight up a steep (no air drag) hill at a given speed. 5.5%, guaranteed. What that means to us causes arguments and possibly reflects mental illnesses, but there's no arguing the number.