Mr Bennet says of Mr Gardiner's letter telling him that a match has been made between Wickham and Lydia: "What is there of good to be expected?"

If Mr Bennet's concern is only for Jane and Elizabeth's marriage prospects then he should regard the letter as good news - as Mrs Bennet does by the way. His negativity about it in the line I quote makes sense to me if Mr Bennet is disturbed by the thought of Lydia's future with such a husband as Wickham. Elizabeth is also distressed by thoughts of Lydia's unlikelihood of achieving future happiness and prosperity with Wickham (chpt 50) and even Jane must comfort herself with the hope that mutual affection will cause Lydia and Wickham to live together happily (chpt 49).