Most successes are unhappy. That's why they are successes - they have to reassure themselves about themselves by achieving something that the world will notice.
Just as you have the instinctive natural desire to be happy and overcome suffering, so do all sentient beings; just as you have the right to fulfill this innate aspiration, so do all sentient beings. So on what exact grounds do you discriminate?
When the suffering of another creature causes you to feel pain, do not submit to the initial desire to flee from the suffering one, but on the contrary, come closer, as close as you can to her who suffers, and try to help her.
The saddest thing in life and the hardest to live through, is the knowledge that there is someone you love very much whom you cannot save from suffering.
There will come a time when everybody will know why, for what purpose, there is all this suffering, and there will be no more mysteries. But now we must live ... we must work, just work!