I've seen people take this lesson too serious and too lightly.

One was a company that refused to ever be accused of requited love. They would look for any excuse whatsoever to decide the other party "wasn't serious." A number of partnerships and buyouts passed them out until they had to be sold in an emergency, and they still acted like everything was totally out of their control up to the end.

And I've also seen lots of "partnerships" where one party really doesn't care at all.

You need to be willing to get rejected, but get rejected fast.