There is no dearth of material on the meaning of life. I found the Stanford Encyclopedia of Philosophy’s entry on “The Meaning of Life” to be a good solid introduction. The Internet Encyclopedia of Philosophy from UT Martin offers an alternative perspective in “The Meaning of Life: Contemporary Analytic Perspectives”.
A large number of writers have attempted to explain why the question of meaning is important, and worthy of study. Viktor Frankl sold millions of copies of his “Man’s Search of Meaning” [an excerpt is available for download from this site – excerpt]. Before the second world war Alfred Adler had written on “What Life Could Mean To You”. Richard Holloway, who was Bishop of Edinburgh and Primus of the Scottish Episcopal Church, has developed a non-theistic view, explained in “Looking in the Distance, The Human Search for Meaning” [an excerpt is available for download from this site – excerpt]. A more modern view of the same human motivation can be found in Irvin D. Yalom’s “Existential Psychotherapy” [an excerpt is available for download from this site – excerpt].
An answer to the question is usefully informed by developments in non-behaviourist psychology. The story can usefully begin with the work of Mead and Vygotsky. Leszek Koczanowicz provides an accessible introduction in “G.H. Mead and L.S. Vygotsky on Meaning and the Self”. For those interested in dipping into Cognitive Science a bit more deeply, Andy Clark’s “Mindware – An Introduction to the Philosophy of Cognitive Science” is a reasonable next step [an excerpt is available for download from this site – excerpt].
One last reference almost demands to be included. Susan Wolf’s “Meaning in Life and Why It Matters” provides a detailed defence of a Subjective plus an Objective requirement for a “proper” meaning for a life [an excerpt is available for download from this site – excerpt]. This book is particularly useful because it includes commentary by four experts who come at their commentaries from different perspectives. She doesn’t reflect meaning or purpose back to environmental contributions, but what she argues is consistent with such a view of meaning.