What were they expecting when they got on boats and went towards unexplored waters? Did they expect to live? Was their some sort of religious imperative that compelled people in those early island nations to leave at some point in their lives? Would there have been regular boats of later settlers after the first group?
Well, remember that Hawaii was just one link in a very long chain--it isn't as though a group of Austronesians woke up one morning and decided to randomly sail across the ocean. The colonization of Hawaii was simply a very late part of a story of the human colonization of remote Oceania that began around 1500 BCE, and during that long time they had gotten quite good at it. I wrote a post some time ago about how colonization actually worked (note: apparently the date I was using for human colonization of Hawaii has been replaced)--it was planned, deliberate, and with numerous precautions to ensure a safe return.
The "why" is a bit more difficult, because we don't have direct evidence. Certainly, there was prestige that went along with founding a new settlement, and the founder could have an illustrious place in mythologized oral tradition (such as Pa'ao for Hawaii--whether or not he was literally historical he demonstrates a "founder" aspect to mythology). This may give an explanation on an individual level, but there are also undoubtedly larger explanations. One is, probably, population pressure--after a certain amount of time on an island, the population may begin to strain the resources. It is true that frequently a given island would be "filled in" long after further islands were colonized, but perhaps colonization of further islands and colonization of the interior would be two strategies pursued by different groups simultaneously--One chief responding to overpopulation might decide to lead out a voyage of exploration, one might lead his people to found a new settlement away from the coastline. There also might be "generational" pressure--a cultural norm may dictate that upon coming of age, the sons "move out" to a new island, not unlike how people are expected to move out of their parents' house today.
This is ultimately a theoretical problem, and different explanations are not the result of different data but rather different approaches, so you can sort of pick whichever suits you. No doubt, the reality was messier and more complex than whatever explanation we can offer. A nice quick overview is Patrick Kirch's "Lapitia and its Aftermath".
hi! in addition to Tiako's comment and link, here are a few more previous discussions
How did ancient people find Hawaii?
How did the Austronesians cross the pacific to places like Hawaii?
