Chemicals such as oxytocin are released, and their desire for commitment and security happens.
That’s one of the differences that happen in that age bracket.
This creates unnecessary pressure, stress, and difficult surprises concerning the other person’s weaknesses or differences of opinions which surface in the future, and the challenge and need of working though the next two or three stages after marriage.
Because not enough time was allowed for significant crises and issues to arise before the two committed themselves to each other through engagement, the couple now discovers each other’s character flaws, different values and perspectives, weaknesses, etc after the engagement and/or marriage.
Is it an inevitability that these stages should occur, though?