This one is tough because it is a lot easier to speak to people you have a relationship with than total strangers! We speak to our customers a lot but they are biased with how we solve their problems. We can also rely on speaking with 'friendly' customers and avoid speaking to customers who are not so happy with us.
Without speaking to people we don't know, we won't get to learn about perspectives of solving a particular problem, and we may fail to learn about other problems that need to be solved.
My advice is to speak to customers and non-customers to obtain a more comprehensive view of your target audience.