I had opposite experience by switching to "eat all the meat you can" protocol from "hardly eat any meat at all" protocol.
There is probably nothing harmful in animal meat per se, except its low in some nutrients (so you have to eat organs or supplement). I find highly improbable that high meat usage can cause disease as most animals have similar evolutionary grounds and if meat protein is not poisonous to them, its not for us too (the same goes for saturated fat). The other thing is how animals are raised, but AFAIK most of the industrial toxins accumulate in fat, not lean meats, and that might be a problem for some people. Highly processed meats should be avoided.
So, you may have problems with
High LDL cholesterol. Its highly genetic stuff and you probably shouldn't be concerned about it. If you are, test it, do not guess it.
High protein load - its very hard to eat enough meat to get protein toxicity, if not impossible, but still its worth mentioning.
If you have kidney problems, lots of protein is not advisable.
Lack of Vitamin C if you are not eating enough plant stuff along with meat (VLC or low carb)
Vegetarian diet is not healthy on the long run, initially it might have positive effects, probably due to caloric restriction which is what most vegetarians probably end with (OK, it might be done correctly but its highly unlikely). First, you risk B12 deficiency (only analogs in plants), and low cholesterol is actually more harmful then higher one. Iron might be problem too (no oxigen and brain suffer first). Vitamin K2 (essential for teeth, proper calcium metabolism, coagulation) On the positive side, lack of protein will induce xenophagy which helps clear intracelular parasites (the same anorexic protocol body engage with some viral infections) so thats why its good on short run. The other positive effects are probably due to general "higher awerness of the food you eat" since beeing vegetarian is more like beeing in sect for most such people then anything else.
Lightness with the meat absence is probably due to failure of digestive system or improper food combining - you need to have good HCL levels in order to digest it well, plus it stays there in the stomach long time after eating. If you remove wheat and/or too much starches (potato, rice) you will fill light again, given that your GI is good - no dysbiosis, h. pylori etc.
I don't know what "more positive outlook of life" means. Is it lack of depression or lethargy? If that was the problem, low cholesterol, B12 and lack of fish meat or oils will make the things worst. Plant industrial oils will do that too (trans fats and industrial toxins) and soy is particularly harmful.
So, the only real problem might be lack of Vitamin C, depending on your other food habits, which you should supplement anyway with pure ascorbate acid powder unless you eat kidnies, adrenals, brainz, and eyes :) Vitamin C is required for dopamine synthesis which might influence your 'outlook'. Lack of it also impairs immune system giving chronic infections way to go. Depression is linked to infections. On the contrary, adding Vitamin C to protein meals will boost its absorption as some of it uses the same receptors as glucose (GLUT4, promoted by insulin, promoted by protein) and without glucose higher rates of C absorption are achieved which may promote infection clearance hence leading to better brain states. Industrial toxicity can cause bad fillings and C is potent toxin clearer. It can also potently fight toxic hard metals from fish meats, with some of them accumulating in the brain and causing depression.
=== EDIT ===
There was additional question in the comment, I believe something like how vegan diet can produce such great feelings biologically.
- As I mentioned, it could be due to xenophagy at least at the beginning.
- Vegan diet is high in carbs. Carbs might have seratonergic effect by boosting L-Tryptophan uptake which is precursor to serotonin.
- Carbs influence the opiate pathway. This means they can reduce pain which leads to happiness. Other then that, they change functioning of dopamine system since simple sugars are addictive (and all are simple, once amylase and friends do their work). Removing them will produce irritation for some time, until dopamine receptors are upregulated. This is actually one hypothesis of obesity and possible explanation of low carb flue. Also, plants might have some not-yet-understood chemicals that influence brain workings since their genome is large and they have to produce bunch of stuff from limited input.
- High w-6 PUFA are boosting anandamide synthesis. This one is responsible for "blissfull" state of the mind (THC from marijuana does the same thing, but better).
- Fasting, which is typically how vegetarian protocol is delivered religiously, at least in this part of the world most of the time means calorie restriction which promotes ghrelin synthesis. Ghrelin is addictive, boosts your intelligence and power, at least for short time.
- Placebo or positive thinking can influence brain state.
- Religion. Thinking that we are supposed to eat plants means that plants are not there to fight for their survival like everybody else but are of divine origin. Happy animal faces in minds of non-animal eaters add to that. I don't mind that, except its irrational - plants are living creatures too, not eating animals will not change anything and life doesn't function that way [if you really want to save animals, evolve enough so you can actually go to other planets and live them alone and/or produce your food synthetically and not starve yourself]. If elephant on Mars makes you happy, I am happy, just don't make me do it or call it scientific.