Honor, family, and legacy
The main idea of the novel seems to be that this novel seeks to defend the honor of those who are suicidally depressed. Although the mentally ill sometimes fail in life in ways that are difficult to understand, Buddy's side of the story is that we should try to extent mercy and charity to one another, because after all, you never really know what someone is going through. This is his service to Seymour's legacy, written in light of his suicide. The latter story is an elegy about the beautiful life of Seymour.
Judgment versus charity
Criticism can be used to find someone guilty, or to find someone innocent. For Buddy, there is so much about Seymour that needs to be taken into consideration, that anyone who dares to judge him based on one moment in time must not be a fair judge of character. He decides that the wedding party is judgmental, because they didn't consider all the factors that they might have considered in Seymour's favor; they only criticized and never empathized.
Mental health and depression
The reason Buddy is so defensive about Seymour is because he knows something about Seymour than many people do not: he knows what an epic person Seymour is, and he knows the weight of the emotional burden that Seymour carries through life (before a time where effective medication was available for the severely depressed). When Seymour commits suicide, Buddy takes a moment to explain to us what we missed by never getting to meet him. He explains what a tragedy this loss of life constitutes by explaining what a magnificent person his brother was. His sister agrees, citing a poem from Sappho about a great, tall husband (on Seymour's wedding day).
