When you read the Buddha's teachings, there are two things you should ask yourself: "Is he speaking the same language as I do?" and "Is he aiming his comments at people of my level of practice?" By clearing up any misunderstandings around these two issues, you will be able to figure out how to apply his teachings to your own life.
For example, with the language of attachment, most people are referring to emotional attachment, which the Buddha analyzed into a whole cluster of positive factors-such as affection, gratitude, trust, and commitment-as well as some negative ones, such as clinging and emotional bias.
While it is said that the Buddha teaches us to be free of emotional attachments, what he actually said is to be free from clinging. When you cling, you latch on to a relationship in the hope that it will bring you true happiness by satisfying your desires and shoring up your sense of who you are. With clinging, even your affection, trust, and sense of commitment can cause you to suffer. It will fuel your emotional bias so that you act under the power of likes and dislikes, delusions and fears, creating even more suffering for yourself and others.
But if you can learn to look for happiness inside-which means straightening out your relationship with yourself-that takes a huge load off your relationships with others. The sense of affection, trust, and commitment you feel for people who are close to you won't cause you to suffer.
As for level of practice, the fully awakened person treats all people fairly and with genuine goodwill. But on the way to becoming awakened, you'll first have to develop wise relationships with the people around you. If your loved ones can't rely on your affection, commitment, and gratitude, how will all sentient beings be able to rely on your goodwill? This is true for both laypeople and for monks and nuns. The Buddha's teachings are full of good advice on how to develop warm, trusting relationships with your family and friends.
As for monks, each newly ordained monastic is told to regard his preceptor as his father. The preceptor, in turn, is taught to regard the new monk as his son. The student monk's relationship to his teacher is literally called "dependence." The teacher's duty is to train the student to become independent within five to 10 years. But the affection, trust, and gratitude of the relationship are expected to last as long as they are both alive and ordained.
So when you look at the language and level of the Buddha's teachings on attachment, you'll see that he wasn't advising against special friends or monogamous relationships. He's just telling you not to cling to those relationships in a way that will cause suffering.