OP: I'm assuming you grew up in the west so it probably doesn't make a whole of sense in the beginning.  I guess I'll try to make it short but I do apologize in advance because it's not a simple topic…

China's history (pre-modernization) is more or less similar (contrary to popular belief) to how life is now.  Most are poor, some are middle class, and the minority (based on the total population) are rich.

In China (as well as quite a few other countries), it's was very important to have a son.  If you didn't have a son, you couldn't carry on the family name.  If you couldn't do that, your family lines after one or two generations basically ended.

If you had daughters, you had to marry them off as quickly as possible because most family kept trying having children till they finally had a son.  This in the olden times produced a problem whereby there was an over abundance of women. That meant men had there pick.  And so they would generally pick the youngest (and most attractive ones) to marry.

Because that mindset has carried through every generation since, it has been ingrained in their nurture.  Thereby having extreme family pressure to be married between the ages of 23 and 25 if fortunate, and anything over 30 being considered "Leftover women".

In many mountain side rural villages it is not uncommon for the pressure of marriage to begin at the age of 18 and there are plenty of stories of 16 year olds being married off (whether it's of their own will or not is another story) as well.

It may still be difficult to understand the exact mentality as we are both men AND foreigners.  However is really is the pressure that most chinese women grow up with.  And if they don't succeed in finding a good husband by the age of 25, their parents start to worry while those who are over 30 begin to looked upon as failures by their own parents.  (Remember that mainland chinese actually care to a degree what their parents think!)

I hope that helps a litte bit.