Just an aside: I find your sentence ambiguous. Some would take it in the sense of "You need more joy/fun/excitement in your life." Others would take it to mean, "You need to believe that everything in your life is going to work out for the best."
The important plural in your sentences is "those." People often misuse "their" as a singular, and "who" may be either singular or plural.
If you wanted to express the same thought in the singular, you might use the singular "whoever" in place of the plural "those." "I feel sorry for whoever lacks faith in his life."
this gets messy because of the gender issue. People (me, for