When I go on a airplane, I get severe pains around my eyes and forehead. Why do I get these pains?

Sinus congestion. You may have sinus and nasal congestion not allowing your sinuses to equalize pressure to the outside world. This will give you a sinus squeeze and pain around your eyes and forehead. Over the counter decongestants (nasal sprays and oral) may be used to treat this. If this doesn't work, you should see an ENT for a full evaluation.
It depends. Many possibilities. Are you an anxious flyer? If so you may be clenching or grinding your teeth and not aware of it. Do you chew gum? Aggressive gum chewing can aggravate the chewing muscles which are located where you describe your pain. Do you have nasal or sinus issues? Intermittent obstruction of the sinuses can cause pressure headache when flying. If a significant problem see an ENT doctor.