The size of your dog doesn't always matter when he wants to sit on your lap. It's easier for you to enjoy a small lap dog as you sit together on the couch, but big dogs often crave the same type attention for similar reasons. Establishing the reason behind the behavior helps you understand whether you need to take action to change the behavior or if it's OK to cuddle.
Just as your pup becomes part of your family, you are part of his pack. Getting higher than your feet allows him to interact more closely with you -- he gets more petting and loving than when he's on the floor alone. When he jumps on your lap and you pet him, you reinforce the behavior and teach him that if he feels the need for some petting, all he needs to do is settle down on your lap. This isn't necessarily a problem; cuddling with your dog makes you both happy. If your dog is too big to fit on your lap comfortably, teach him to sit on the floor and reward him with petting instead.
You add stability and security to your dog's life, so it's natural he would run to you when he's not so sure of the situation. Getting up on your lap allows him to feel comforted and protected from strange dogs, people or places. Comforting your pup in this case might not be such a good idea. It can reinforce his behavior, making him incapable of dealing with new situations without your comforting hand. Allowing him to address new situations on his own, not on your lap, can help create a more confident pooch.
In a pack mentality, there's always an alpha dog. This should be you, meaning you have the first right to food, water, toys and beds. When your dog jumps on your lap and demands attention by barking or pushing his head under your hand, he might be trying to assert dominance over you. If you have more than one dog, he might be trying to show dominance over them as well. Teaching him you're the boss might include turning your back on him when he wants affection and giving him attention on your schedule instead of his, taking his food bowl away for a few minutes before he's finished eating or winning every time you play tug.
When your dog sits on your lap as you cry or when you're sick, he's likely offering you comfort. He can't tell you how sorry he is that you are sad or don't feel well, but many dogs can sense emotions and illness. Getting close to you makes him feel better, so he thinks letting you get close to him makes you feel better -- and he's probably right. However, the motivation might be a bit more selfish. When he sits on your lap in front of other people or dogs, he might be marking you as his own so no one else can steal you. In his mind, he's warning others away from you by sharing his scent, which helps keep you safe.