Hmm, this is a tricky question to answer and it may branch to a variety of topics, so I will just answer in a straight ago reward approach.

For me, I would start something that I can't pursue and accomplish. What I said is applicable to all major decisions in our life like a Degree, Business, Marriage and so on. And these are the things we can not aimlessly start since it might lead to regrets that are difficult to reverse.

Of course, some people would say how can we know if that decision in our life will be successful if we don't even begin? We'll, I never said "don't do", what I'm trying to say is "don't start until a person have enough reasons to pursue it". So that's it :)