-The Strawhats are, of course, the first true friends Robin has had
-Every group or organization Robin has been associated with in the past has been destroyed for one reason or another (usually because the government has been hunting after her), so naturally she believes the same will happen to the Strawhats.
-And perhaps the most important reason, CP9 is threatening to destroy the Strawhats with the Buster Call. The Buster Call is the ultimate form of attack by the Marines, in which a fleet of ships utterly decimate any given target, be it a pirate ship or an entire island, without prejudice. And as you'll soon discover...