Sentence:  The nice guy, they say, finishes last. But in romantic relationships, the <WSD>nice guy</WSD> often isn't even in the running."..." The <WSD>nice guy</WSD> may have expressed an interest in dating you"...". The <WSD>nice guy</WSD> is the person who you trust and feel comfortable with, but don't see as "fun" or "challenging" or really all that "interesting," other than as a friend, of course. 
Meaning of the word marked with <WSD> tags:  An inoffensive adult male who desires romantic companionship but who is too bland and uninspiring to be attractive.