From interested +‎ -ness.

interestedness (uncountable)

1. The state or quality of being interested, or having an interest; interest
2. selfishness.

   Synonyms: me-firstism, self-centeredness, self-obsession; see also Thesaurus:selfishness

   • 1748, [Samuel Richardson], Clarissa. Or, The History of a Young Lady:[…], volume II, London: […] S[amuel] Richardson; […], →OCLC, page 214:

     I might give them what degree of credit I pleaſed ; and take them with abatement for Mr. Solmes's intereſtedneſs, if I thought fit

• “interestedness”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.