Sorry, I assumed your data would be in a computer-readable format. Try this:

wordsByDate4= 
 Dataset[Function[<|"date" -> DateObject[#[[1]]], 
       "words" -> ToString /@ ToExpression[#[[2]]]|>][
     StringSplit[#, " "]] & /@ 
   StringSplit[StringReplace["2015-08-21 {Spain,gains,balloons,tomorrow}
               2015-08-21 (England,loses,rocks,after,rain)
               2015-08-21 {Brazil,gets,bananas,and,abandons,red,spinach}    
               2015-08-21 {Israel,begins,sleeping,but,loses,race}
               2015-08-21 {Japan,fights,at,noon,might,win,large,gains}
               2015-08-21 {Russia,rejects,koala,adopts,on,lonely,tiger}",{"("->"{",")"->"}"}], 
    "\n"]]

First we'll define the dictionaries:

negativeWords = {"exclude", "fights", "lonely", "loses", "reject", 
   "tiger"};
positiveWords = {"adopt", "balloon", "banana", "embrace", "gain", 
   "joy"};

Then we'll extract the actual data from the table you posted. Since I assume your data is in a computer-readable format, you'll end up doing this a bit differently.

wordsByDate = 
 Dataset[Function[<|"date" -> DateObject[#[[1]]], 
       "words" -> ToString /@ ToExpression[#[[2]]]|>][
     StringSplit[#, " "]] & /@ 
   StringSplit["2015-08-21 {Spain,gains,balloons,tomorrow}
    2015-08-21 {England,loses,rocks,after,rain}
    2015-08-21 {Brazil,gets,bananas,and,abandons,red,spinach}    
    2015-08-21 {Israel,begins,sleeping,but,loses,race}
    2015-08-21 {Japan,fights,at,noon,might,win,large,gains}
    2015-08-21 {Russia,rejects,koala,adopts,on,lonely,tiger}", "\n"]]

And we can define a function that created a list of the positive, negative, and sum sentiments given a list of words. It's easier to operate on a string than a list of words, so we convert the list to one.

sentimentList[words_List] := 
 Block[{lowercase = StringJoin[Riffle[ToLowerCase /@ words, " "]], 
   positive, negative, sentiment}, 
  positive = StringCount[lowercase, positiveWords]; 
  negative = StringCount[lowercase, negativeWords];
  sentiment = positive - negative;
  {"positive" -> positive, "negative" -> negative, 
   "sentiment" -> sentiment}]

Then we can execute sentimentList on each row of the table:

wordsByDate[All, Append[#, sentimentList[#[["words"]]]] &]

Here's the result: