The themes in Something Borrowed (in the midst of this CRAZY love triangle between Rachel, Dex, and Darcy) are friendship, jealousy, and (most importantly) the dangers of passivity.
The friendship in question here is that of Rachel and Darcy. They have been friends since they were young girls, and are still best friends when they become adults. However, as readers, we learn throughout the novel that the "real" friend here is Ethan, Rachel's friend who lives in England. Always there to listen. Always there to tell the truth to. Always gives perfect advice. Rachel and Darcy ultimately betray each other in that both of them sleep with the love interest of the other. The friendship is ultimately doomed from the start when we realize that, even in grade school, Darcy always lauded her achievements over Rachel and "stole the show."
Put simply, Rachel realizes through the story that she is jealous of her "friend" Darcy. Darcy has always "stolen the show" away from Rachel from elementary school on. At the beginning of the book, though, Rachel doesn't realize this. It is Darcy's relationship with Dex that is the catalyst. There is one part of the novel that is a vast retelling of Rachel admitting her jealousies: Darcy stole Rachel's boyfriends, Darcy acquired all the attention, Darcy mooched off Rachel when out of work, and ultimately Darcy stole Dex away from Rachel.
This is why you should never, ever get your hopes up. This is why you should see the glass as half empty. So when the whole thing spills, you aren’t as devastated.
It is Ethan that explains to Rachel that she may have originally make the "mistake" of sleeping with Dex because of her jealousy.
The Dangers of Being Passive
The problem about the list of all of the jealousies above is that RACHEL DOES NOTHING ABOUT THEM. She buries them deep inside. Over half a lifetime, she eventually sleeps with Darcy's fiance, Dex. Rachel was passive in elementary school. Rachel was passive in high school. Rachel was passive in college. Rachel is passive, ... until NOW. It is only when Rachel ceases to be passive and DEMAND that Dex make a decision that things start to happen. Dex originally says that he can't hurt Darcy, ... but then realizes he can't deny his true love: Rachel.
I love him wholly and unconditionally and without reservation. I love him enough to sacrifice a friendship. I love him enough to accept my own happiness and use it, in turn, to make him happy back.
At the end of the book, Rachel is rewarded for finally being an active participant in life. Dex and Rachel are able to be a couple together in public and enjoy their love.