Yeah often times it's not avoidable (the condition or psyche) of an individual. All you can do is be a friend like you are to any one, support them and hopefully influence them to be looked at by specialists. But yeah sometimes with no signs it's hard to know. I knew a buddy who was the rowdy one of the bunch and he ended up asking his sister to go with him to a specialist. He was happy, a go-getter and never really backed down from a challenge. Mental Health is an issue getting a lot of attention, especially in a time where people's minds get easily jumbled around with distractions.