This depends on how far you believe the influence of a god goes. Did the god create the Universe and set up the laws of physics and then let nature take its course, or does he/she/it have an active involvement in everything with nothing going unnoticed or uninfluenced; or any position between these extremes.

In the latter case, why create species and then destroy them (such as extinctions)? What is the reason for creating species that survive by killing and eating other species (that feel pain)? Why do we have diseases that cause so much suffering and what is the point of allowing humans to find cures for diseases that god created?

In the former, science can not explain the causes of the Big Bang or the creation of life, but that does not mean there is not a scientific explanation. Many things that were once thought to be to work of god are now explained by science, such as earthquakes, plagues and eclipses. 1,000 years ago god would have been an adequate explanation of these events, but we now have other (scientific) explanations. The things that science has explained so far do not need the intervention or guidance of a god; therefore if there is a scientific explanation of the creation of the world it will probably not need a god either. If there is no god there is no reason (ie nothing is planned or meant to happen).

Just because science can not explain something now does not necessarily mean we never will be able to understand it. I feel a little uncomfortable saying that because something is extremely complicated and we don’t understand it, it must be attributed to a god.

I’m not saying we should discount the existence of a god completely; I just find scientific explanations more convincing and do not see the need to believe that things happen for a reason.

You're assuming things happen for a reason. Could it not all be chance?