Button would never join or have joined McLaren, due to the following reasons:
1: he would need to work hard and work under a lot of pressure - something he isnt used to or prepared to do. This is why he chooses to stay in teams that are at the back of the grid.
2: he would've had to take a pay cut, compared to big salary he was getting paid by Honda.
3: every mistake he made would come under the microscope, which is what happens when you are driving in a championship winning car.
4: everytime he had a poor race/qualifying, it would be highlighted and scrutinised.
5: at the end of the season, his points total would be compared with Hamilton's and we all know that over the last 2 years, Hamilton has scored more points than any other driver in F1, making it statistically difficult for Button to get close. Mentally, before even signing on the dotted line to drive with Hamilton, he would be petrified as his entire reputation could be shot to pieces 9 months later (should Hamilton blow him away).
6: everything Button did would be compared directly to Hamilton and as we all know Hamilton is something very special - even the driver widely recognised as the best in F1 (Alonso), was unable to outscore Hamilton, in the same car.
Button would have absolutely nothing to gain and everything to lose...unless he believed that he could beat Hamilton, in which case he would relish the challenge, but we all know that isnt the case.