Why do some people advise against eating meat?

It is best to fill your mind with saatvic thoughts. The food you eat and your mind are directly related. Eating meat products induce ‘rajasic’ qualities in your mind. That may be the reason why some people advise you to abstain from eating meat.

What are rajasic qualities?

Anger, lust, greed, tendency to indulge in worldly pleasures, resorting to any means to fulfill this end is worldly rajas.
Even in divine life, there are rajasic qualities. Doing divine activities with great pomp and show, hurting the body in form of severe penance, exhibiting and projecting one’s Sadhanas to outer world, etc. are examples.