Not an atheist, have a question.

[Angrboda]

No, the pen doesn't have to fall. That it will fall is a supposition based on an assumption that the laws of nature are uniform, and will continue to operate in the future as they have in the past. (We can reason that they have behaved similarly throughout various past times because if they hadn't, the universe we see today would be different than what it is.)

Your questions about life are best answered by saying that we don't know specifically why the types of life we witness today are the only types that we have evidence of existing. It's hoped that once we have better ideas on how this kind of life started, we may have better ideas about why it alone exists, but until the riddle of abiogenesis is solved, we simply do not know.

As to the rest, the specific type of life that exists has certain requirements for it being here and flourishing, one of which is the presence of heavy elements, which didn't exist at the beginning of the universe, as they only form from the remains of prior stars. Thus it only exists in certain environments that either historically or contingently were required for them to flourish, without which they would not do so. (One speculation is that life on earth originally formed in the vicinity of deep sea thermal vents. This is just to point out that the relationship between current life, and original life, is not necessarily direct or obvious.)