Mr. Collins doesn't buy Lizzy's rejection. Its no wonder - he considers himself a good catch - a clergyman with a decent income; Lizzy has no fortune and may not do any better than him; that she's just being coy by refusing him. With these advantages, he thinks - how can she refuse, even after she rejects him about 4 times. There's too much presumption or ego here but the society in which they lived didn't help much either.