Well, having a seventh element of harmony sounded great to Twilight… turns out, they already know the bearer of the 7th element. Almost everypony knows her, in fact. And most of those ponies… lets just say, they wish they didn't know her. All Derpy ever wanted was a friend, she never wanted a shiny necklace with her cutie mark on it. But hey, the necklace could lead to finding six new friends who would stay by her side. Maybe being the element of hope could lead to a new life for her. A new start. Derpy wasn't going to ruin that chance, no matter what.
She accepts the 7th element, determined to start a new life. What she gets is certainly a new life, just not the life she wanted…. being the 7th element bearer is a lot harder than expected.