To peer with the eyes partly closed
To have a tendency or inclination
An inclination to something
The condition of not having the visual axes parallel
The glow of fire made him squint as they neared the field.
The man you are interested in may squint his eyes and smile for all the reasons you have listed in your question or he may do this behavior because he thinks it is cute and endearing.
They frequently have poor visual acuity and are extremely sensitive to light (photophobia), which causes them to squint in ordinary light.