Since you asked,
Girls do not go because they have nothing else to do. They do not go because they are commanded to. They do not go because they feel obligated or pressured to. They do not go to learn lessons for themselves, because even though they hope they will change and learn much, they could definitely learn those same lessons through doing equally good things at home. They do not go to have fun. They do not go because they are part of the lonely hearts club. They do not go to tour other parts of the world. They do not go because they think it's better than getting married and raising a family, because it is definitely not. They don't go to become more marketable in the dating world, either. They don't go because it's an easy little break, they actually make a lot of sacrifices to be able to go. They don't go because all their friends are doing it.
So, why do thousands of girls go on missions every year, when no one expects them to? Why do they agree to go anywhere in the world, possibly learn a foreign language, go out of their comfort zones, leave their families and friends and their personal lives behind, and obey rules for a year and a half?
They go to share the gospel of Jesus Christ. They go because they want to and they feel it's right for them. They go because knowing the truth is so incredible they can't keep it to themselves. They go because they love the church. They want others to feel what they feel and to know the precious truth they know. It doesn't matter that it may be hard for them, because it's not about them.
They go because the truth they have is a pearl of great price. It changes their daily life, and they can't imagine making sense of this world without it. They go because the gospel makes them so happy that they want to share it with everyone they know, even those they don't. For instance, they're grateful to know that their family can be together forever, and they want all families to know that. They are grateful to know God lives and answers prayers, and that is a truth that everyone deserves to know.
They go because the worth of souls is great.