Imagine you are sitting at a and your friend is sitting at d, imagine in your and your friends world, there is gravity on both the ends the top end and the bottom end, now you and your friend share a special property that if you decide to jump in upward direction or fall in lower direction, you will shake hands with each other
If you are able to say that you have find such a friend, then that friend is your complement
Now try to relate this story to the definition of GLB and LUB and if you and your friend both GLB and LUB coincide, then my friend
Congratulations, you have found your complement