I think you may be missing the point here. It's not don't think about whether you should or should not and plan as much as you can accordingly – your ambitions. I think it's actually don't expect you can know how you will feel – your aspirations. And more importantly don't deterministically tell yourself how you should feel about what you aspire to become.
Allowing your aspirations to more flexibly become whatever the situation requires is important. As described in the finish with respect to the time between we aspire to do something and work toward those aspirations:
> “what happens in the meanwhile is also life.”
Life gets in the way, and as people we certainly can change for good and bad.