Question: Why is Beetlejuice so intent on marrying Lydia? Why not just find someone else?
Answer: If he marries a human, he can leave limbo and become human again. He needs to get away from his wife, who needs him to complete her blood ritual.
If he just wanted to marry a human, any human, he would not have waited for Lydia. He kept a picture of her on his desk.
Answer: Why are most people so intent on marrying someone? He loves Lydia. In the first movie, she was a teenager, and he mostly needed a wedding to get out of his punishment. In the years since, he appears to have developed more respect and appreciation for her. He stopped her wedding to Roy, exposed Roy's fraud, and transformed their almost-wedding into the smaller, "private" ceremony that she really wanted. He doesn't want to "just find someone else" for the same reasons that many people don't.