Link exchanges happen naturally. And some times unknowingly.
I’ve unexpectedly found links pointing to my site from those I’ve linked to some time in the past when creating a resource for my visitors.
No notification or any type of request asking for a reciprocal link was sent; it just happened.
Some times it can be a good thing to just link to a site because you’re impressed with the content or it’s a good resource for your visitors.
You don’t have to notify them that you’ve added a link to their site, but you can. I prefer not to as the reason for linking to them in the first place was to build a better resource, not to exchange links in an attempt to boost your link popularity.
Excessive reciprocal linking with sites just to boost your link popularity are not good and it’s a practice I recommend that you avoid.
Feel free to link out! Build a better resource. And to hell with nofollow; sometimes the more you give, the more you get. 😉