I want to give money because I want to help a certain cause, not because I've been guilt tripped into it by being shown fly covered starving babies or abused kittens.
Unfortunately chuggers work, they've realised they get a lot more money back from a relatively small outlay, and also, once they have got people signed up to 3 or 5 pounds a month or whatever, for most people, its just not worth the hassle to get it cancelled, so they have a constant flow of many many small amounts of cash flowing in.
Generally, if I'm feeling like I'm being forcefully pushed into donating, I'll find a similar charity to give to instead, I don't like being emotionally manipulated. Similar reasons to why, if I'm in a relationship, I'll downplay valentine's day, she'll get a card and candy maybe, but I'm not going out of my way to do as I'm told by Hallmark. I'll treat her well all the time and exceptionally occasionally at random so it feels special instead of expected.
I do however love charity shops, my love of books is pretty much solidly fed by these and libraries, only a few authors get my money for new books. (OMG PIRACY etc)