"Nice Guy Syndrome" really isn't a real thing. It refers to a narrative, usually perpetuated by men who find themselves placed in the position of "the friend" to women they are interested in (often referred to as "friendzoned"), and speaks to an accompanying bitterness that speaks to a feeling that as a "nice guy" they are not sexually interesting, whereas a "Bad Boy" might have an easier time attracting others. This is, of course, hugely reductive, and is at the centre of many Incel points of view, who feel it is the fault of others that they do not have a partner.