He lost his best friend in Splinter Cell 1 When he was shot by a militant.
In Splinter Cell Chaos Theory he had to shoot his friend, while on a mission, he found out Shetland had gone corrupt.
Then Splinter Cell Double Agent he found his daughter was dead when her death was faked in the later Splinter Cell Series. They put Sam Fisher through allot of suffering & pain :O. His daughter was the only person he had in this world, everything ripped apart from him in a New York minute.
Imagine being Sam Fisher you stepping into his shoes if a person had to experience What he had to experience through out his life this dramatic psychological experience he went through :O.