My DH and I quote this phrase to each other often, because it is so true -- that looking forward to future happiness is itself a source of great happiness, one to be savored for itself. I think this is an insufficiently appreciated insight, beautifully phrased by JA.

I don't think JA is condemning happy expectation in any way, though it does indeed set people up (Marianne obviously) for a fall if their expectation is disappointed. I hadn't noticed how Elinor is contrasted with Marianne on this point, and shown to be sometimes too cautious or pessimistic to allow herself to expect -- and therefore enjoy -- happiness. I think this is an instance of Mrs. Dashwood being right about Elinor (for once).