There is an old (supposedly Chinese) saying, which, paraphrasing, goes something like this:  "One without smiling face should not open a shop."  While many a business owner takes that too far, with their demeanor bordering on that of a 'happy idiot', it is nonetheless true that, in general, people prefer to deal with, and are more trusting of, uptone people.  Partly that is because uptone people are easier to communicate with.  They 'accept communication' more readily than, for example, someone who is "too serious" or "depressed" or "angry".

Since a referral is essentially the transfer or forwarding of a communication line/communication channel onto you, before someone is going to be willing to refer their communication lines to you, they have to be comfortable communicating with you, and (so the story goes) feel comfortable that when they 'hand off' their communication line to you, you will handle it well.

It is NOT just about the quality of past work done.  A lot of, lawyers for example (I can say this, as I am a lawyer) are very effective in Court and doing whatever they do, but they are terrible at communicating effectively with their clients.  So, while the clients might like the result, they may not be willing to refer any of their communication lines to the overly terse lawyer.  If, however, the lawyer is approachable, duplicates his clients' concerns, does not invalidate them at every turn and is generally uptone and accepting of communications, her or his clients will refer new clients to this lawyer EVEN IF this lawyer 'lost' the case.

In the end ... as someone said in a movie ... you're 'always closing' ... which, if interpreted in a positive way means ... no matter where you are, or what you are doing, it is your demeanor and your ability to invite communication that will determine the quantity and quality of referrals.