Being human.
Want to know the worst part about being human? Feeling. That is honestly the worst aspect and yet it's the most important.
When you meet people in life and you become close with them you develop these feelings. They don't even have to be feelings of love or hate just as simple as caring for them. For me this is where it get's dangerous in a friendship. Why? You invest so much time and energy into that person and sometimes it turns out to be the best decision you could've possibly made and other times you could not have been more wrong.
Be careful who you invest your time in.