The most obvious, and most important, reason to engage with others about veganism is to mitigate the harm and injustice done to (nonhuman) animals. There is, however, an additional reason to talk to people about ethical issues of this kind, namely out of a recognition of the moral integrity of others. By choosing not to converse with others about an injustice as serious and as glaring as the present one, we to some extent degrade them and regard them (even if not explicitly) as lacking the requisite sympathy and judgement. We assume that they will not care or understand, and hence, we assume something about their character.
To be sure, many people do not care or understand, even after the relevant arguments and evidence have been presented to them; but there are many people who would be appalled by the actual state of things, and who would cease to support such things if they only knew. Accordingly, there is a certain amount of guilt that comes with passively watching people engage in practices which they themselves would disapprove of if they were fully informed, not least because of the injustice that is otherwise allowed to continue unchallenged.
As vegans, do we not all think from time to time, ‘I wish someone had told me sooner about this‘? There are undoubtedly many people who would feel the same way but have never been adequately informed. Just as most would consider it wrong not to inform others of the harmful effects linked to some activity directly concerning their wellbeing, it is arguably wrong not to inform others of issues which they care deeply about, but lack the knowledge that would provoke the appropriate changes in their life. In order to avoid this, it is also important that we do not dilute the position we wish to communicate. What we owe to other sentient beings, and to other human beings, is to provide the unadulterated truth. By presenting a weak, and ultimately specious, version of veganism, we increase the risk of our listener someday thinking, ‘I wish they had been clearer about such an important issue!’, and they could rightly censure us for fundamentally distrusting their moral judgement.