There are many factors that affect handwriting.
Environment: Environment can affect children in a number of ways. Lack of opportunity to play and experiment with graphic materials will have developmental consequences. Pressure from parents and teachers to perform can cause undue tension. Other factors include inadequate light and poor position of the lighting, the type of writing surfaces (too many things on the desk), and the position of the writer to other people (too close together).
Motor: Fine-motor control is an essential foundation for writing skills. Without it the accuracy necessary to guide the pencil through the required formations will be severely affected. Motor planning is required so that the child can accurately reproduce the required shapes.
Vision: Difficulties with vision will affect the child's hand-eye coordination which is required for aspects of spatial and motor control. Poor vision is often the cause of many learning difficulties, so adults should be sensitive to emerging patterns.
Perception/Spatial: Handwriting has many spatial requirements such as direction, size, shape, slope, and positioning. Spatial problems can make it very difficult for children to create a legible print. Visual discrimination is required to distinguish between similar and mirror letters. Auditory discrimination is necessary to associate verbal information with graphic forms.
Attitude: A poor attitude and an inability to care or take responsibility for one's own actions is arguably the main reason for children's poor handwriting. Children must be trained to take responsibility--and parents and teachers must expect it. Self helplessness is a behavior some children develop in handwriting and in other areas if they are not encouraged to become responsible for themselves. Boys seem to be the main culprits.
Visual-Motor Integration: This refers to the child's ability to accurately reproduce shapes. It is the process that links what the eye sees to what the hand produces. Without adequate development in this area, the child is going to have difficulty reproducing the complexities of letter formations and their spatial requirements.
Attention/Memory: Without attention to a task, a child will remember little, if anything, and any attempts to learn are futile. It is known that young children respond best to new and colorful stimuli. Because of their short attention span, it is important that our attempts to teach children handwriting are interesting and sessions are shorter.
Emotion: Our emotional state can affect our handwriting dramatically. For example, if a child feels uptight and under pressure, it is reflected in his or her handwriting. We cannot expect children to be robots and produce the same quality of written work every day. As their emotional state fluctuates, so may their handwriting.
Excepted from Ready-to-Use Fine Motor Skills & Handwriting Activities for Young Children / Joanne M. Landy and Keith R. Burridge / The Center for Applied Research and Education / 1999