Sir Toby and Sir Andrew are knights. More than likely they are former knights who have turned to continual drinking to pass the time. When Toby's friend shows his disappointment that Olivia has not looked his way with any interest, Toby reverts back to what a knight would do in his day--fight for her! A show of manliness for a woman, or dedicating a fight to a woman, was considered honorable. Not only that, but acts of valor and sacrifice for a lady love would be sure to catch a woman's attention. Hence, Sir Toby must pick a fight for Sir Andrew in order to make his friend look good and to make Cesario look bad in front of Olivia. The following is by Sir Toby:
"Why, then, build me thy fortunes upon the basis of
valour. Challenge me the Count's youth to fight with him; hurt him in eleven places; my niece shall take note of it; and assure thyself, there is no love-broker in the world can more prevail in man's commendation with woman than report of valour"(III.ii.28-32).
It is interesting that the plan must be to "hurt him in eleven places," but Fabian backs up Sir Toby by declaring to Sir Andrew that the only way to get Olivia's love and attention is by winning a fight for her. With a little peer pressure and appeal to manhood, Toby hopes to persuade his friend to fight to get Olivia valiantly from a the young boy.