No, he's not the only reason.

VOA: standard.other

"The lives of our soldiers are being put on the line because of our own security and that is the only reason that a government in any country would put our soldiers lives in danger."

VOA: standard.2009.10.21

If a woman says no,the in-laws can take her children and prevent her from inheriting anything, even if the only reason she does not want to marry her brother-in-law is to keep from infecting him with HIV.

VOA: standard.2009.10.21

Lincoln had only one reason to fight: to save the Union.

VOA: special.2009.10.08

is the concept of man as a heroic being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute."

VOA: special.2010.06.06