Many emotions aren't really caused without the assistance of another, but aren't always excluded to. Like depression a lot of the time is caused by loneliness, guilt, fear, etc. But I'd have to say that fear is a major one. Many emotions we experience are caused by fear. Such as one person fearing that another is becoming more popular and gaining more reputation than he is, he might boast and increase his pride to overcompensate for that, possibly making him feel back on top. Or the fear of a horrible event happening in your household (possibly family issues) which causes you to fall into depression, leading to loneliness, hate, revenge, etc. Or the fear of somebody trying to steal your gf/bf could cause you to outburst in anger, paving the way for revenge, hate, pride, etc. Love is also powerfully dangerous, as it can cause (has proven to cause) people to enter states of insanity and kill others. But it has also wrapped and gathered people under one flag to defend a nation, such as early American defending from Great Britain, showing love for ones country and opportunities.