What's the biggest lesson learned from doing marketing?

I'm still learning, but so far: quality is WAAAY more important than quantity. Reaching a select few relevant people is much better than reaching thousands, who are never gonna buy your product. I'm sure it's not true to every product, but with B2B marketing I feel like this is a golden rule: try to focus your marketing activities on smaller groups.

Marketing is about making a deeper connection and keeping in touch.

I learned the biggest lesson is "Make your customers the hero of your stories."