Because if you make friends casually, you may make more friends and meet people who can become bosom friends.
But don't you think it's good to make friends casually?
Friends are divided into good friends and bad friends.
Making friends is heart-to-heart and can help you when you are in trouble; When he (she) is in trouble, you can think of him (her) wholeheartedly. Those are friends, but those you casually make are not real friends.
Be sincere in making friends. You make friends casually, and you make a lot of "friends". Who else can you be sincere to?
As the saying goes, lack is better than abuse!