There's no hard and fast law. Is visiting friends a valid reason? Sure, whatever you consider valid, as long as it's legal under US law, is up to you, it could be sand grain counting if you wanted.
What it comes down to, and what we say to a lot of people asking on here, is that it is up to the immigration officer on the day's discretion.
That sounds scary but he/she is following a series of training on making sure your interests and statements are valid and true. So if you say tourist visa (fine, your VWP can reset by returning to UK), your ESTA is up to date, and you have accommodation evidence, plus your return flight dates, that's going to satisfy most of them. He just wants to see if he believes your story. So if there's anything he's unsure on, he'll ask more questions. Simply be prepared.
Oh you're returning to work? Do you have a manager we can call to confirm this? Oh you're the boss - a partner/employee? Can you provide evidence of say, an upcoming client meeting? Do you have a copy of your rental agreement / a recent utilities to show that you're still living there and are therefore likely just on another holiday?
A series of questions just to clarify stuff. If you're not trying to weasel and are literally just visiting friends, and have all your documents (flights, accommodation readily at hand) then they're satisfied faster, and you're on your way.
If something doesn't check out, you get more intense questions. If it fails or a lie is detected, they may THEN refuse entry.
You always have the option to speak to a team leader or manager and sometimes get this fixed. But your options diminish very quickly if you're caught lying. So it's easier to have the paperwork all ready before hand and just show them the docs.
Or, as more likely - they'll be "tourist? how long? * stamp * enjoy your trip" and you'll be through ;)
