Well, I suppose they had to hook someone up with Charlie and it's not like anyone else was available - at the time (Roman was leaving, Hugo was going to get with Martha and Miles was with Kirsty). Plus, I think the producers took on board fan feedback about them looking 'hot' together - during their brief no-strings fling...and, therefore, it was always the plan to put them back together, from day one of Angelo's return, no matter how contrived it seemed. Angelo also needed a life, outside of that lame 'Mystery' they created as an excuse to bring him back for, to stop him looking even more purposeless.
Yeah, though I'm not that into the show's constant unlikely romances.. I don't mind Martha/Liam...and