When Bruce wayne returns as Batman in Dark Knight rises he isn't the same person on terms of his responses and agility which he possessed years ago when he decided to hang up his boots at the end of Dark knight (plus the limping at the start of the movie)  . Alfred strongly believes that he is also inferior in physicality, strength and reaction to Bane when he sees all those footages of him during the Motorcycle chase scene he enacts in front of Gotham.

Alfred tries to explain the scenario to Bruce wayne to the extent of Bane's strength and also believes that Bruce wayne is trying to be batman to satisfy his obsession, even though he is old Bruce wayne still has the heart of the old batman and tries to counter him stating that his weakness can be exploited which ultimately leads Alfred to leave on the fact of their heated discussion.

All those years Alfred never gave up on Bruce because he believed that he could fight those odds in reality but in the case of Bane the age alfred instinctively knew that this was the fight Batman can never win at that point of time given his strength and also earlier Alfred hints to him (the coffee place he talks about) about starting a family (or finding the right girl) which could've led him to that decision

(Rightly so alfred is correct at that point and Bruce wayne gets his broken back immediately after he leaves)

When Bruce Wayne returns as Batman in Dark Knight Rises he isn't the same person in terms of his reaction times and agility which he possessed years ago when he decided to hang up his boots at the end of Dark Knight (plus the limping at the start of the movie). Alfred strongly believes that he is also inferior in physicality, strength and reaction to Bane when he sees all the footage of him during the motorcycle chase scene he enacts in front of Gotham.

Alfred tries to explain the scenario to Bruce Wayne to the extent of Bane's strength and also believes that Bruce Wayne is trying to be batman to satisfy his obsession, even though he is old Bruce Wayne still has the heart of the old batman and tries to counter him stating that his weakness can be exploited which ultimately leads Alfred to leave on the fact of their heated discussion.

All those years Alfred never gave up on Bruce because he believed that he could fight those odds but in the case of Bane, Alfred instinctively knew that this was the fight Batman can never win at that point of time given his strength and also earlier Alfred hints to him (the coffee place he talks about) about starting a family (or finding the right girl) which could have led him to that decision.

(Alfred is correct at that point and Bruce Wayne gets his broken back immediately after he leaves)