Paul Seymour has an article titled “How the proof of the strong perfect graph conjecture was found?“, which is an informal and rather nice documentary-type article, and gives a high-level description of the process of finding the proof of the strong perfect graph theorem.

In Section 7, “What’s left?”, he writes

Having worked in Berge graphs for three years now, we have developed intuitions and skills that

took a long time to grow, and also a great fondness for the graphs themselves.theUnfortunately

main problem is solved, and there is a cold wind blowing, almost as if it’s time to go and work in a

new area … There was one other really nice question: what about a polynomial time recognition algorithm? Can one decide in polynomial time whether a graph is Berge? Is the question in NP? These were still open … We thought it would last us for another three happy years, butits resistance collapsed after just a couple of months, and Maria and I managed to twist it into an algorithm.sadly

(Bolding was done by me.) My point is that, Seymour was happy as long as there was an interesting problem to work on, and as soon as it is solved, the happiness is gone! While this may seem contradictory to a non-mathematician (who might think that the mathematician becomes happy after he solves a problem), it is SO TRUE. A mathematician has the best feeling in the course of solving the problem, and maybe a little while after solving it, but not any later!