Because we are in it together.
To fail to love and to fail to forgive are part of the same thing, which is rejection of reality - the reality that we are called-upon to choose salvation and not just for ourselves; but by participating in the web of love, to accept the gift for mankind.
To love and to forgive is precisely this participation in active and consenting aspects.
(Always remembering that love of neighbour must be second or subordinate to love of God, which enables it; and forgiveness is an attribute of our own humility before God. Yet for many, perhaps most, love of God is attained via love of another, or some others - and the same for humility and forgiveness. The particular opens out into the general - seldom vice versa.)