Attachment is not a good thing, neither is complete detachment.
You have to use your ability to watch (as done in vipassana meditation) attachment and understand it, how it is an impediment for your growth spiritually and otherwise.
For it is an illusion that the person (or the thing ) to whom you are attached makes you happy, makes you soulful. There is so much fuss about Soul mates, soul mates might be true but you have to realise that attachment whatsoever will hamper you in the pursuit of real happiness.
Attachment makes us happy but that happiness is delusional, real happiness is unconditional, real happiness is not only presence of happiness it is also absence of suffering and every conditioned things are of impermanent nature.
For understanding suffering because of attachment, we can meditate on impermanence. Everything that arises out of certain causes and condition(which means everything, every being) are impermanent. Humans can comprehend only the gross impermanence, to understand the subtler impermanence, we have to meditate on momentariness, which is an outcome of impermanence, it says every thing that arises because of certain causes and condition are changing every moment.
So if you are attached to someone(or something) at the present moment that someone after some moments, is not the one you got attached neither you are the same person who got attached.
So everything is of changing nature. Nothing is permanent, every thing is dependently originated.