Here are some handy principles, that strike me as being a generally good way to treat other people. In fact, they also strike me as a pretty good guide to customer service*.
It should be assumed that everyone has the capacity to make his or her own decisions, unless it is proved otherwise.
A person should have all the help and support possible to make and communicate their own decision before anyone concludes that they lack capacity to make their own decision.
A person should not be treated as lacking capacity just because they make an unwise decision.
Actions or decisions carried our on behalf of someone who lacks capacity must be in their own best interests.
Actions or decisions carried out on behalf of someone who lacks capacity should limit their rights and freedom of action as little as possible.
In fact, they're the principles I've been given to read about my duties under the Mental Capacity Act 2005 as I sign a Lasting Power of Attorney. They just feel like overall pretty good principles - don't assume other people are useless until they prove themselves to be, and then don't take advantage of them. And if someone does something you think is stupid? That's not necessarily a sign that they're stupid, they might just think differently to you.
Sometimes the law isn't an ass after all.
* For some reason, and I fear it is occasionally related to being female, I find myself being treated by salesmen and tradesmen in direct contravention of principles 1 and 2. Why must people assume that the fact I lack a Y-chromasome also means I lack the ability to understand mains electricity, or the internal combustion engine? I'm sure it must get tedious dealing with people who don't understand your specialist field, and goodness knows I'm aware of the frustration of clueless customers, but I do at least try and give my customers the benefit of the doubt before making decisions in their best interests after they demonstrate a complete lack of understanding of ionisation cross-sections.