"for one thing, ..." means "one reason is ...", often used when there are several reasons.

Hello, “For one thing...” introduces what will be a list of reasons or results. “To come to do something” means there has been a change or evolution in thinking or behaving that has started the new thing. You could also say “We ended up thinking that...” Hope that helps! Best, Martha

There are two parts to it that don't need to be used together. "for one thing" and "we have come to think that" "For one thing" is a sentence starter that introduces an idea. It is often used when two or more ideas are going to be presented. The idea after "for one thing" might be followed up with "For another ..." "We have come to think that" means that our thinking has changed and now we think the way that is described after that phrase.