Lots or really nice gear and good thinking.
I have looked for years and can only find two statistical studies of wilderness fatalities. One was a study of fatalities in western National parks. One was collected data for New Hampshire SAR cases. Different areas but very close results. About 75% died of almost equal quantities of heart attack, drowning, and falls. Hypothermia and hyperthermia were statistically far less insignificant - 1-3%, or in the range of gunshot and motor vehicle accidents. (One issue not addressed was the extent to which the apx 25% who drowned did so after hyperthermia weakened their ability to stay afloat, thus being a contributing cause. However, the safety lesson is unchanged.)
The numbers suggest more attention might be paid to physical conditioning, physical exams, and knowing one's limits; safety around water; and fall prevention. Many publications on survival in the wilderness (such as the B.S.A. Wilderness Survival Merit Badge pamphlet) neglect all of these topics. Few publications cover them all.
As for a "positive mental attitude," realistic thinking acknowledges risks while not giving way to panic. A positive attitude has it limits and is not a substitute for minimal gear any more than gear alone can cure panic, ignorance, or excessive optimism. (A good many have died of terminal optimism. The "Gold Rush" and western migrations are full of examples.)