There are two sides to a plan; making it and then accomplishing it. However, there is a big gap between both, and it takes will and determination to make that leap to cross from the former to the latter.
I think being "not too serious" with a plan in the first place has an underlying effect of inhibiting one from achieving it.
It may be because the plans are unrealistic that humans will not achieve them eventually. Realistic goals are the ones that are within our potentials and capabilities.