The characters of Paul and Gertrude Morel in D.H. Lawrence's Sons and Lovers are highly autobiographical. Lawrence, himself, had a relationship with his own mother which he mirrors in that of Paul and Gertrude.
Gertrude, who is a firm, strong-willed and intelligent woman is also a woman who feels dejected and unhappy with her own life. As a result, she vicariously lives through the lives of her children whom, as males, she considers to have been born for bigger and better things than herself.
The problem is that Gertrude has a tremendous amount of energy which rubs off on her children, making her smother them with her care and protection. This takes away from the normal balance that should exist between a mother and a child, where roles are delineated and boundaries are respected. The result is that Gertrude literally interrupts the normal emotional growth that Paul inevitably needs to go on his way to maturity.
If we were to label the relationship between Paul and Gertrude in psychological terminology, we could easily declare Paul as a man who suffers from Oedipus Complex. This Freudian concept suggests the uncanny affection for a parent of the opposite sex and the dislike for the parent of the same sex. Paul displays these emotions verbatim. He detests his father and seems to be in a sort of competition with him. The same thing happens later on in his life when he sees in Baxter a challenger and also a father figure. Back to his oedipal side, however, Paul's passion and dependence on Gertrude borders on the inappropriate because of the obvious need that Paul has for his mother's perennial presence, for her touch, and for her support.
Ironically, Gertrude suffers from the same problem because she obsesses over Paul, hates his girlfriends, and manipulates his emotions. The mutual dependence definitely makes their relationship hard to categorize under any "normal" relationship precisely because their synergism results in that none of them can move forward. In not so many words, the relationship between Paul and Gertrude is abnormal in that the need for each other's presence is in no way indicative of a healthy or nurtured relationship. If anything, it represents a relationship that is enmeshed and amorphous. When Paul applies an extra dose of morphine to his mother at her deathbed he is not only trying to speed up her death, but he is finally opening the door to his freedom. That is how negatively dependent his life was to the life of his mother.