I think there's some confusion here. The colonization of Sicily and the Sicilian Expedition are two very different things. The first is a gradual process of settlement and conflict with local populations that was initiated both from Carthage and from Greece around the late 8th century BC, with most major settlements in place by the mid-600s BC. The second was a military expedition sent out by Athens in 415 BC to turn the Greeks of Sicily into tribute-paying members of the Athenian Empire.

The Sicilian Expedition was not, therefore, about the conquest and colonization of a new land, but about the subjugation of other Greeks in a known and prosperous part of the Greek world. Indeed, the Athenians had sent smaller forces to Sicily at least once before, in 427 BC, to interfere in a local conflict. They knew perfectly well what they were getting into.

That said, the Athenians were apparently quite excited by the prospect:

All alike fell in love with the enterprise. The older men thought that they would either subdue the places against which they were to sail, or at all events, with so large a force, meet with no disaster; those in the prime of life felt a longing for foreign sights and spectacles, and had no doubt that they should come safe home again; while the idea of the common people and the soldiery was to earn wages at the moment, and make conquests that would supply a never-ending fund of pay for the future. With this enthusiasm of the majority, the few that liked it not, feared to appear unpatriotic by holding up their hands against it, and so kept quiet.

-- Thucydides 6.24.3-4

We do not know how the rest of the Greeks felt about it, but presumably they would have been displeased at the thought that yet another part of the Greek world would be put under the Athenian yoke. Many of the cities of Sicily were inhabited by Dorians, having been settled from Corinth; other Dorian communities such as Sparta would not have been happy to see the Athenians planning to subjugate their people.

As for the initial colonization of Sicily, we do not know if the Greeks were excited about it, because writing was not in use at the time.