An underlying theme in both As I Lay Dying and To the Lighthouse might be that life is disappointing. In each case, we have a group of people whose interactions are strained and confused, and who face inevitable loss as a result.
In As I Lay Dying, Dewey Dell expresses this feeling of deep disappointment and loneliness at life.
It's because I'm alone. If I could just feel it, it would be different, because I would not be alone. But if I were not alone, everybody would know it. (p. 58)
She feels lonely because she needs an abortion and has no one to talk to, because she can’t tell anyone. She tries to buy medicine to have an abortion, but the druggist refuses. Anse takes the money should was going to use for an abortion to get a haircut, further demonstrating the ironic disappointment of life.
In To the Lighthouse, each person also feels isolation and disappointment. There seems to be a tug of war between wanting to be alone and craving the independence of solitude.
Waves were all round them, tossing and sinking, with a log wallowing down one wave; a gull riding on another. About here, she thought, dabbling her fingers in the water, a ship had sunk, and she murmured, dreamily half asleep, how we perished, each alone. (ch 11)
The characters are often desperately trying to get others to see their point of view, and their reality.
In each book, we have a group of characters who do not communicate well with the other characters. The reader is the only one that truly knows what is going on and understands them.