Or "why do my pictures not come out where I want them?"
Do yourself a *huge* favour and stop worrying about this! When the work is absolutely finished in every other respect you can fiddle with figure and table placements but it is an enormous waste of time during the writing process.
The 'fix'
It is usual to put figures and tables in as 'floats' which means that they have no fixed position with respect to the text. LaTeX finds the 'best' place to put the figures according to some algorithm that you need not worry about.
If you disagree with the placement you can override it. But this causes its own big problems so wait until you have done all the hard stuff before you get bogged down in the trivial stuff.
Then read this ► and this ► (which are both excellent) and then read the documentation on the following packages:
\usepackage{float} here ►.
\usepackage{placeins} here ►.
These tips and settings and packages will give you most of what you need by way of 'control' over placement of floats. There is, however, no magic bullet.