No single food (except breast milk for infants) contains everything humans need. Eating the exact same meals daily means you get the exact same micronutrients — and miss the exact same ones. Deficiencies take months to develop but are invisible until symptomatic. Example: same salad every day = same low zinc, same low B12.
Eating the same food daily increases risk of developing IgG-mediated food sensitivity. This is well-documented with eggs, dairy, and wheat. Rotating foods reduces this risk. If you suddenly develop symptoms from a food you've eaten daily for years — this may be why.
Different fibers feed different bacteria. Eating the same foods = feeding the same bacteria. Microbial diversity drops. This is linked to inflammation, poor immune function, and even mood changes. 30+ different plant foods per week is the target for gut diversity.
This is the BENEFIT. Not deciding what to eat saves mental energy. Steve Jobs wore the same outfit daily for this reason. If eating the same thing helps you maintain a healthy diet, the benefits may outweigh the risks. Just rotate through 2-3 different breakfasts, lunches, and dinners — you still get the simplicity with more variety.
Aim for 10+ different plant foods per week (minimum). Rotate through 2-3 different breakfast options, 3-4 lunch options, 4-5 dinner options. Your "same thing every day" can be a rotating set of 3-4 meal templates with different vegetables/proteins. This gives you the simplicity AND the variety.