What's actually causing it
The forearm muscles that curl your wrist and fingers and turn your palm down all anchor onto one small bony point on the inside of your elbow, through a shared tendon. Every time you grip, twist, pull or lift, it takes load, and enough of that without enough recovery changes the tendon 1.
The word most people expect here is "inflammation". But in most people with elbow tendinopathy the tendon is not inflamed or swollen 1; what's changed is the quality of the tendon tissue. That's why icing and resting takes the edge off without resolving it.
The drivers are ordinary: gripping, writing, lifting, pulling and twisting 2. In a large general-population study, repetitive movements and forceful activities were each independently associated with medial epicondylitis, and so were smoking and obesity 3. High cholesterol, a high-fat diet and diabetes also affect tendon health 2, part of why two people doing identical work end up with different elbows.
Day to day that means tenderness on the inside of the elbow, difficulty gripping, and trouble twisting things like a jar lid 2.