Expectation is the root of attachment.  Relationship with expectation is attachment and relationship without expectation is detachment.  That attachment is inseparable from suffering is a universal truth based on logic and experience.  All enlightened beings have no attachment and this is why they are blissful and beyond all suffering.  Where there is less attachment there is more happiness, and vice versa.  Everyone knows this truth on the mental level and in terms of their own direct experience.  Other creatures apart from humans generally have little attachment and for a short period of time, either in order to survive, or to protect their young.  However, when their offspring become self-sufficient, the parents usually withdraw and at the same time lose their attachment.  Humans, by contrast, tend to build up numerous and often unnecessary relationships with almost anyone they meet.  It is the reason also why they make many friends.  But all this shows that they are empty inside, that they are unfulfilled by themselves.  Yet, if a person can become detached from everyone and can live happily alone, that individual comes to have universal love and compassion for the whole of humanity as well as for all living beings without any expectation.