Appearances can be deceiving, it is true. And, of course, it's easier for me to be cynical of the motives of someone whose policies I almost universally disagree with, and be more trustful of those who are closer aligned with my world view. As for any other spectator.
It isn't always about whether or not I agree or disagree with the politician, though. Margaret Thatcher stopped being PM when I was still less than a year old, so I can't possibly evaluate her except through the prism of history. I think she was wrong on a great many issues, and the less said about her last couple of years in office the better; but I'd still hold that she became PM because she cared about the UK and wanted to fix its problems, as she saw them. I trust her motives, in other words. On the other side of the coin, Ed Miliband only ever made leader of the opposition rather than PM but I simply couldn't trust him or take him seriously, even if he was more or less "on my side" more often than Cameron was.
Those two examples hardly prove anything, but there we are. I try my best to be fair and objective, and almost certainly end up failing most of the time.