Is it bad if I don't feel like talking?

Not necessarily. It can be good to have periods of introspection, and not everyone is by nature chatty -- thank goodness! It's also probable that you can learn more by listening than by talking.

If your mood is also low and you are preoccupied by negative thoughts, that could indeed indicate depression. That's part of normal ups and downs in life, and it can also be a reaction to specific stressors in your life, including nutrient depletion from a poor diet or increased stress. If you are concerned, talk to a friend, and if that doesn't help, consider consulting a medical professional.
Is getting a six pack the key to having a lot of sex?

There is no correlation between six pack and sex.Neither sex pack symbolises FITNESS LEVELNow as you have talked about abs and sex ,so you must be worried about performance effect being having abs or not .First of all let me tell your performance and sexual activity matters on following factorsHormonal imbalances and low levels of testosteroneSmokingExcessive

What can I do for 10 minutes every day that would improve my mathematical ability?

It might sound strange coming from me since I have trouble doing my own math! But, not long back, my maths professor had said that "Mathematics isn't something you can learn. It's a skill to be mastered." And, as far as I know, you can only master a skill if you practise