It's not about being rejected every single day. It's about learning to accept rejection more easily, which it sounds like you're doing fine on with part-time canvasing. (Or perhaps you could cope easily before that.)

I'd take a bet that you're not the type this was designed for. For some, being rejected at all is a huge blow to ego, confidence, and self-esteem, which can have serious effects on mood and productivity, for instance.

For these people, getting rejected more often means that each rejection is less significant and reinforces the knowledge that rejection happens. That is much healthier, and I'd hypothesize that it can increase one's average level of happiness. (It also gives an individual much more control over himself -- or takes it away from others, at any rate. Same thing.)