1) They are not always as they seem, so give them a chance.
Us volunteers don't know the histories of the animals we work with at the shelter. We don't know where they've come from. Sometimes I assume that certain dogs will be aggressive due to their size, their breed, or the shelter's designation of them as "high energy" dogs. I've learned, however, that "high energy" sometimes just means "strong", or "jumpy", or "loud", or "I'm going to be really excited to meet you, but please don't be alarmed if I get in your face or bark at you". In my experience, it has never meant "aggressive". To be honest, some of the biggest dogs I've met have been the most patient, and some of the rowdiest dogs in the kennels are cool as a cucumber once they get outdoors. Point is, you really never know who you're dealing with until you give them a shot. Allow yourself to be pleasantly surprised. And if instead they play too rough or try to nip you, that's okay. Just put them back, walk away, and know that you tried.
2) We need to be grateful for simple things and cherish simple desires.
These dogs are well-fed and well-sheltered, get medical care and at least minimal daily TLC. But do you know what these dogs get excited about the most? Being taken outside. Their greatest joy is going outside, and they're more than happy with just that. Can you imagine people being like this? Not just content, but ecstatic about being outside! When was the last time you felt grateful for mobility? When was the last time you acknowledged the gift of being able to go out as you please, rather than being stuck inside a room that might not even have windows?
3) Make the most of the occasions you have to interact with people. Give them all the goodness you can, because this might be the first and last time you ever see them.
Some dogs stay at the shelter for weeks or even months, so we volunteers become used to seeing them and develop relationships with them. But for volunteers like me who only go once or twice a week, we might work with a dog once and then never see them again because they've been adopted out. We might've anticipated seeing them again or trying to get better at handling or understanding them. But in the end we don't get that chance because we have no control over who will still be there when we return. So we have to make the best of the little time we have with them: make this walk the most fun it can be, make this belly rub the most satisfying it can be, make this respectful distance between us as calm and appreciative as it can be.