Anse Chastanet is very secluded and very much a couples place, although I'm sure there's no problem with singles and friends staying there too, but you might feel more comfortable in the Rodney Bay area. Right on the best beach on the island, Reduit, and within walking distance of tons of restaurants and bars. I would recommend Rex or Royal St. Lucian, Coco Palm is great (although not right on beach, but only a 5 min. walk away). There's many other hotels in that area too.

But I guess it's all what it is you're looking for in a vacation. If you want a private, secluded setting, then Anse is good for that. But if you want a great beach and close to other activities that you and your friends can do, Rodney Bay is better for that. You can always do a day trip to the south side of the island to see the Pitons, rainforest, waterfalls, etc. (either by boat or car).

Hope this helps. Let me know if have any other questions. I have been to SL many many many times.