Mr. Bennet gives two classes of reasons for allowing Lydia's ill-fated trip to Brighton Beach in disregard of Elizabeth's heartfelt and rational reasoning against such a trip. The first class relates to Colonel Forster and the second to Lydia herself. First, though, it is well to point out that Lydia did not "go alone" to Brighton; she went under the chaperonage and in the companionship of Colonel and Mrs. Forster [this points out the need for close reading and close attention to syntax and punctuation when reading Austen (... in general)].
1. Of Colonel Forster, Mr. Bennet says that he is a respectable man and one who, in his authoritative position over the regimental soldiers, is in a good position to keep Lydia in line so that the folly she might commit will be the least disastrous kind: "Colonel Forster is a sensible man, and will keep her out of any real mischief ...." In other words, Mr. Bennet feels that Colonel Forster's position and inner character make him a suitable chaperon to a young female folly-finding rascal like Lydia. As it turns out, he was wrong about what Lydia could manage to do despite a reasonable male chaperon (perhaps because of a female companion as foolish as she herself was).
2. Of Lydia, Mr. Bennet's comments have less to do with reasonableness and more to do with the jaded observations of a disillusioned, distanced parent (which is what Elizabeth eventually faults him with). Mr. Bennet says of Lydia that:
- Lydia will agitate in her vanity for the attention of men until she has publicly shamed herself in one way or another: "exposed herself in some public place."
- her folly in Brighton, under the chaperonage of Forster, will be the least expensive and inconvenient to repair than any other Mr. Bennet can imagine: "... with so little expense or inconvenience to her family as under the present circumstances."
- by implication, one has to expect to be "connected with a little absurdity" to be worthy of real regard (another sign of his disillusionment and how he copes with his own disappointments): "youths as cannot bear to be connected with a little absurdity are not worth a regret." [He speaks satirically, yet genuinely.]
- Lydia is too poor; her dowry will so inadequate that she will not be really attractive to very many young men (except the insincere sort who may be tempted to entertain themselves with her without serious regard to her welfare) and that "officers will find women better worth their notice."
- in wider social company, she may learn to view herself rightly rather than with such unmerited vanity (such unmerited pride) and learn a more realistic and rational view of herself (like Elizabeth herself learned to know herself better following Darcy's letter): "her being there may teach her her own insignificance."
- (this is the most disillusioned statement of this disappointed man) there would be no peace at Longbourn estate until the vain, prideful, silly young girl would get her own way: "We shall have no peace at Longbourn if Lydia does not go to Brighton. Let her go then."