This week, I met a few people who assumed that I was reluctant to share my ideas of what I want to build, a few of them even assumed that there was a possibility that I would want them to sign an NDA before I would share them.

I guess a lot of people are super guarded when it comes to sharing business ideas, which is super strange to me because:

Sharing an idea and watching someone else's reaction is such a great way to gauge how cool and/or good the idea is.
Sharing an idea with a stranger is a great way to practise and polish how you will pitch the idea.
Sharing an idea is a great way to get feedback.
If an idea is really so easy to steal, execute well, and profit from — so much so that you are unwilling to share even a tiny bit of it — it's probably an indication that it has a extremely low barrier of entry. In that case, even if you're first-to-market, if you can't turn a justifiable amount of profit faster than competitors can dilute your market share by copying, it doesn't seem like something worthwhile getting into in the first place.
- Following from the above, I suppose there is a possibility that your idea could somehow be a legal (hopefully also ethical) way to make a quick buck followed by a quick exit, and somehow nobody has thought about it yet. Well, isn't it better to not let anyone know that you are working on it at all? There is always a chance that people can figure out through your experience, affiliations, and so on.
For someone to steal your idea, it requires that, to name a few, they actually think that it's a good idea and better than any ideas they may already have; has absolutely no qualms with "stealing"; understand the problem you want to solve; actually knows how to go about solving the problem; has the time, resources, and energy to see it through. That's just a few things that I can immediately come up with, and I haven't even mentioned anything about the market.

So, yeah, ask away! Happy to tell and no NDAs needed!