Sometimes people tell you they are there for you, but in reality some could not. "A friend who lies for you may also lie against you.” Just knowing that could be true makes you not want to trust anyone at all.
Of course, it's impossible to not trust anyone but it also makes you live in fear. You also wouldn't have many friends at all. Without trust there would be no friendship relationship. It is true, not every friend of yours is against you. Besides, if they are against you then they were never really your friend in the first place. You really have to see the first month or two how they are before you can really tell the difference. For some, even more than a month.
You really have to be careful on who you choose as a friend. You can't just go out there, point at someone, and say “They will be my friend”. If you do that you're only putting your self in danger. Don't make a friend decision based on your loneliness either. You should first look at people with your common interests and then see how much you can trust them.
I'm sure you've heard of situations where you try to be friends with someone and they make you do all sorts of horrible tasks. Now would a friend make a “friend” do tasks for friendship qualification? I think not. Don't forget those situations where the so called “friend” gets jealous over you and tells you false information that will only get you in for trouble. Now see, those are not true friends. They are people who use people for their own selfish benefit. Those are the kind of people you need to watch out for.