Section 21 advice needed

Possession - Section 8 and Section 21

This has probably been covered a million times before but I'm a bit confused about the whole section 21 business. I am renting a flat out on 14th June on 6th month AST but not sure about whether or not to issue a SEC21(B) after they move in as I'm rather hoping that they stay on after the 6 months (assuming all goes well). If I issue the SEC21 will that not give them the impression that they have to find somewhere else once the 6 months is up and therefore I could potentially lose out on a good tenant.