Labour could not now be elected if people voted only out of self-interest. The Labour movement was based on economic justice (the value of labour) but on the reality that if we got universal suffrage then it was in most people's self interest to give us pensions, sick benefits, a health service out of tax and a national insurance system, and that peole would contribute according to their means.
To some extent the philanthropy of the rich that gave us public buildings (town halls, libraries, baths) and parks was replaced by fair taxation (why ask the rich for a donation when you can tax them instead so that the tightfisted contribute as well as the philanthropists).
Any vestige of that Tory paternalism went out of the window with Thatcher. But under successive governments (and in the EU) Britain has become richer - so rich that we can't afford to maintain those public buildings and parks. Were a rich benefactor to offer us his lands for a park we'd say no as we can't afford to look after it.
Austerity (ideologically motivated as Duncan Smith pointed out) has made the country a mess. Public squalor just to save a few quid on council tax (itself a Tory fraud after their poll tax debacle). Were this many people sleeping in shop doorways 7 years ago? Labour had virtually eradicated rough sleeping.
If self-interest is just about money in your pocket to spend on stuff you don't need, then with current average wages there aren't enough "poor" people to elect Labour (for the many not the few is a good slogan but it doesn't have the same resonance as Shelley meant after Peterloo).
For a Labour majority you have to have enough people who recognise the public good as in their interest and not just their own self-interest. And that includes the evidence that more equal societies are happier societies. Not of course that there is such a thing as society - the Tories may disavow Thatcher's statement but it's the same message.