Key point: She's just about to finish her education and begin her career. She's still young and idealistic. Of course she's excited about her work! Of course she can't wait to begin doing all the things for which she's prepared for years, and of course she can't imagine wanting to stop. And I do hope she'll love it, but as time goes by, she will discover the negatives of her job -- not that she won't still like it overall, but she will discover that no job is fun all the time.
If you've stashed away money, you're set for the day that her job is no longer worth it -- for the day that the bad moments begin to outweigh the good ones. I like my job too, but free time is more fun.
Also, if you stash away money now, you're set in case one or both of you NEED to quit. Things happen: Sickness, accidents. A smart person will earn money (and save and invest) while he or she is young and has the opportunity. Not all of us have that opportunity forever.
Finally, stash away money now so that if she wants to stay home with children in the future, it'll be a choice.