what was his reason for separting jane and bingley?

Darcy's claim was that he separated them because he saw Bingley had a partiality for Jane unlike any Darcy had seen in him before but he did not detect in her any symptom of peculiar regard. Interpreted a bit for the modern person, this means that he was afraid Jane really did not love Bingley, and wanted to separate the two before Bingley was so deeply in love that he would be hurt. Marrying a woman who really did not care about him would certainly have been worse than unrequited love, but either was to be avoided.

Following this assertion, Darcy talked of the family's want of connection, the situation of Mrs. Bennet's family, the want of propriety betrayed by both parents and all three of the younger sisters. He asserted that any connection with such a family would have been most unhappy.