I heard the following idea from my Rosh HaYeshiva, Rav Isaacson. 

What does being thankful really mean? The first time that thankfulness is expressed in the Torah is by Leah; not by Adam and Chava, nor Noach, Avraham and Sarah, Yitzchak and Rivkah. Why? 

There are two forms of being thankful. The first is easier and the one which most, if not all, of us do: give verbal thanks. That’s when something good happens and we thank HaShem for it happening or if someone does a favor for us and we thank them for it. The second, obviously more difficult and less common: living thankfully. It occurs even when something bad happens or someone inconveniences us in someway and we feel thankful for it. The different is saying thank you and feeling thanks.  

Now we can somewhat understand what it meant when Leah expressed thanks. Up until now, she assumed that what she was experiencing was all part of the process. There were 4 wives ad they were all going to have 3 tribes each. Once she had the 4th tribe she changed her outlook on life, and instead of just saying thank you she felt thankful for everything in her life. Leah didn’t have it easy at all - being married to a husband that explicitly loved her sister more than her, she felt horrible. Only when she had Yehuda did she change her mindset. 

In life, we will experience things as good and as bad. The only way for us to be happy and get through it is to realize that we are a small part of the big painting, and that painting is beautiful. What is something that you are thankful for that doesn’t come off as good?