Some possible reasons as to why Alan Milburn (Lab, Darlington 1992-2010) has decided to go and work for David Cameron. Not all of these are necessarily mutually exclusive, and not all of them are necessarily intended as criticisms of the former health secretary.
(i) He genuinely sees Cameron as the 'Heir to Blair' and sees the coalition as a Blairite continuity administration carrying out the same kind of public service reforms which he (ie Milburn) was prevented from pursuing in government by Gordon Brown and his allies. He certainly would not be alone in this view of recent political history. There is evidence that Cameron himself sees it this way.
(ii) He genuinely believes there is a need to tackle the slowdown in social mobility and sees himself as the best person to do this. Though doubtless so does IDS, which should make for some interesting policy discussions.
(iii) He wants to put two fingers up to Brown and Co for sidelining him within the Labour Party and failing to make more use of his ideas on social mobility in the run-up to the last election. If so, this is hardly surprising. I myself advocated on a number of occasions that Gordon should set aside his personal loathing of Milburn and take on board some aspects of the agenda he was putting forward. It could have greatly helped in the task of "renewing" New Labour intellectually that Brown ultimately failed to accomplish.
(iv) He wants to launch a new political party positioned somewhere between the Lib Dems and Labour. Okay, I don’t really believe this, but stranger things have happened and by joining a Tory government, the Lib Dems have left a bit of an opening in the market for a new centre-left grouping, though if he wins the Labour leadership, I would expect David Miliband to move fairly swiftly to plug this.
(v) Ego. Always a consideration where Milburn is concerned. But perhaps, on this occasion, not necessarily a deciding one.