Until Winston falls in love with Julia he is able, more or less, to outwardly conform to the Party and keep all his rebellious thoughts hidden. Of course he is under no illusion that eventually he will be caught even so, but when he falls for Julia his rebellion inevitably becomes somewhat less discreet.Mr Charrington's shop, where he meets her secretly, also becomes the place of his capture, as Mr Charrington turns out to be a member of the Thought Police.
By falling in love, Winston goes against all the Party stands for, as he thus gives way to warm, natural human emotions and instincts, which the Party aims to wholly suppress, except the artificially-induced emotion of love for Big Brother. The Party wants to completely control human beings by controlling their emotions; therefore falling in love is Winston's true rebellion, even more than all his intellectual musings.
Winston's life is ruined by falling for Julia, in the sense that this directly leads to his capture, and ultimately their rebellion is not successful: their love is ruthlessly beaten out of them, they are hideously manipulated and they become just another couple of Party drones.
However, in another sense, Winston's life could actually be said to be enhanced by falling in love and thus experiencing natural human emotions, even if only for a short time.