I don't know how old you are, but the most likely explanation is simply biological changes. Thermo receptors in the human body, most noticeably at hands and feet, become less sensitive as we get older, or, to put it another way, the threshold at which we experience these sensations is much higher, and that's an ongoing process as we grow and mature. There's a clue in this, for example - the reason why you're advised to test the baby's bath water with your elbow, even if you are in your twenties, is because the baby's receptors are on full alert and the skin is much more vulnerable than older skin, so your hands, at 25, won't experience great heat when a baby's will. There's a complicated scientific explanation regarding nerve fibres, but I won't go into that. I'm assuming this may also apply to the tongue, particularly given that we lose taste buds over the years roughly as follows - you may start out with thousands (some people have higher numbers at birth), but around age 80, the average human has about 80 left.
Regarding the lack of tears - I don't cry with onions any more either, although I still get the burning sensation. The reason I don't is because I've got something called Sjogrens, and am now rarely able to actually produce tears, no matter how much I may feel like it. I doubt this is the explanation for you, or your tongue would be too sore to put up with hot, spicy food, but there are other conditions that cause less fluid to be available to the tear ducts, again related with ageing, so your eye may, first, be less sensitive due to nerve fibre changes, and secondly, the tear response is not so readily activated even if a perceived irritant is detected.
Not very cheerful I know, and yet more proof that the obsolesence factor is built in...