Customers are seldom disappointed by today's service in to itself
They tend to get upset when today's promises don't materialise tomorrow. Worse still - when their situation changes today and you don't adjust your service tomorrow - they will also be disappointed.
Disappointment comes because a promise made today sets an expectation that exceeds what is delivered/received tomorrow.
In a long term relationship we often see this manifest itself as businesses continually focus on 'new business' over that which they already have - foregoing promises made to existing clients in days/weeks/months or years past in lieu of wooing new clients to them.
Remember, tomorrow is where you deliver what you promised today - you are in control of your clients disappointment today!