Blessings during a meal

I want to thank Hashem for every food that I eat. So, why is it that I’m not allowed to make a blessing over other foods, and I am only allowed to thank Him for the bread? I want to express my gratitude for everything. Also, am I allowed to add words of my own to a blessing? For example, can I say “thank You for making such a beautiful and tempting tomato, so full of flavor?  

The sages, divinely inspired, determined, how best to thank the Creator for every type of food. You have to remember that even the most detailed expression of thanks will never be able to equal the quality of gratitude expressed by the words chosen by the Men of the Great Assembly. They also knew the right way to express appreciation for every meal. However, after you recite the proper blessing and begin to eat, you can express your gratitude in your own words, with as many details as you want for as many foods and other enjoyable things as you want. And this is very important, along with the exact wording of the blessing that was formulated by the Men of the Great Assembly.

The sages knew that eating bread is especially satiating, beyond the feeling of being full. And therefore, it was fitting to express this with a special blessing.

Shulchan Oruch, section Orach Chaim, chapter 177