What are the causes of pain and pressure in the left eye?

See an eye doctor. There are many dozens of possible reasons, some more dangerous than others. You need to see an eye doctor sooner rather than later.
Many. Pain and pressure feelings in an eye can be a sign of a myriad of conditions from minor annoyances like dry eye, sinus disease to serious conditions like glaucoma, uveitis and infections in the eye. You must seek an ophthalmologists opinion for this.