This question is posed by skeptics who, presumably, do not believe heaven or hell is real. If heaven is supposed to be so good, to the point of utter bliss, how can people enjoy this knowing that people they love on earth are suffering in hell?
As with most questions like this, it can be either a good question or a illogical criticism, depending on the motivation beheind the question. If it is posed as a legitimate question from a person who is curious for the answer, it is a good and fair question. If, on the other hand, it is posed as one more criticism from someone who, even if they were given a reasonable answer, would still reject the Christian faith and merely move on to another criticism. Such people are generally not looking for answers, but are merely looking for things to criticize.
First, if this is a criticism aimed at trying to disprove something, it fails. Such a question does not prove anything, and certainly does not make any factual statements about the truth of heaven or hell. It could be the case that heaven and hell are both real, regardless of whether people in heaven think about loved ones in hell.
Second, the answer is rather straighforward. Heaven transcends earth by a margin so great as to be difficult for us to comprehend. The descriptions in the book of Revelation that describe heaven must continually make comparisons to things that we know. The apostle Paul mentions a man, presumably himself, that was taken to heaven and heard things that were unutterable to mankind. So the ways of heaven are above the ways of the earth.
Think of a simple illustration. If people lived in a two-dimensional world, they would not comprehend a third dimension. When they moved to a three-dimensional world, it suddenly becomes difficult to comprehend how a two-demensional world would be like. Another simple illustration. If you are in a dark room, and someone shines a flashlight in your eyes, it hurts your eyes so much you must look away. But if you take that same flashlight out into the bright noonday sun, you can not only easily shine it into your eyes, but the sun overhwelmes the flashlight to such a degree that in order to even tell if the flashlight is working, you must cup your hand over it to see if the bulb is lit.
Heaven will work in a similar fashion. The things that we care about on earth will suddenly become rather puny and not worth worrying about. What will be much more important is what we learned in heaven, the new light of God, in a place where there is no night.
The ancients referred to Christians as ultimately being able to see God, and experience they referred to as “the beatific vision.” When we see God for who He is, and grasp what gifts He now has for us, then the cares of the earth will no longer matter to us. Hence in heaven, there will be no crying, no marriage, and we will be able to worship God directly.
This is nothing new to those who are Christians today, for we know that to worship Jesus directly would be greater than any human relationship we now have.
As an aside, this is also the answer to the question about pets. Will our pets be in heaven? Think of it this way: all our desires will be fulfilled, so if you desire your pet (or any other loved one), you will have it. But in reality, you will have a new set of desires so far above the earthly ones that you will no longer care about earthly relationships.